Streamflow Alterations, Attributions, and Implications in Extended East Rapti Watershed, Central-Southern Nepal

Streamflow alteration and subsequent change in long-term average, seasonality, and extremes (e.g., floods and droughts) may affect water security, which is a major concern in many watersheds across the globe. Both climatic and anthropogenic activities may contribute to such changes. Therefore, this study assesses: (i) Streamflow and precipitation trends to identify streamflow alterations in the Extended East Rapti (EER) watershed in central-southern Nepal; (ii) relationship of the alterations to climatic and anthropogenic sources; and (iii) implications of streamflow changes to the socio-environmental system. The trends in streamflow were analyzed for pre-and post-1990 periods considering the abrupt shift in temperature trend in 1990. Results showed a general decreasing trends in discharge post-1990 in the EER watershed. Human activities have significantly contributed in altering streamflow in the EER. Human-induced streamflow alterations have affected the water availability, food security, river health, aquatic biodiversity, and groundwater abstraction in the EER watershed.


Introduction
Water security is a major concern in many watersheds across the globe [1]. Climate change and human-induced activities have made the issue of water scarcity even worse. Climate change/variability, coupled with the ever-increasing population and limited availability of freshwater, has added challenges in securing water for various uses; both human and environmental [2]. Climate change has influenced freshwater systems and their management through impacts on water quantity, quality, and timing. Furthermore, climate-induced exacerbations of the hydrological cycle have increased or intensified the magnitude as well as the frequency of extreme weather events (e.g., floods and droughts), which may have contributed to the variation in the streamflow. As a consequence, it is increasing the concentration of pollutants, and sediment transport mechanism, endangering the aquatic life, and aggravating the spread of water-borne diseases. Human activities such as land use/cover changes and a concurrent increase in water withdrawals for various uses have reduced water availability. This trend worsens the issue of water scarcity in the future. Therefore, changes in freshwater availability, either due to climatic or human-induced factors, could impact the wellbeing of human societies through implications in multiple dimensions, such as social, economic, and environmental. Therefore, assessing alterations in water resources, their attributions, and implications are imperative to inform the design and implementation of strategies for safeguarding water and food security. Average annual rainfall in the EER watershed, based on data at eight meteorological stations, vary from 1750 mm (at st905, Daman, Elevation = 2312 masl) to 2365 mm (at st906, Hetauda NFI, Elevation = 474 masl) ( Table 1). Analysis of rainfall at Rampur (Station Inde = 902) shows strong seasonality in the rainfall, with about 80% of the average annual rainfall is available only during four rainy months (June-August). Similarly, average monthly maximum temperature (Tmax) in the EER watershed varies from 22.0 °C in January to 35.9 °C in June, whereas average monthly minimum temperature (Tmin) varies from 7.7 °C in January to 25.4 °C in August. Average annual rainfall in the EER watershed, based on data at eight meteorological stations, vary from 1750 mm (at st905, Daman, Elevation = 2312 masl) to 2365 mm (at st906, Hetauda NFI, Elevation = 474 masl) ( Table 1). Analysis of rainfall at Rampur (Station Inde = 902) shows strong seasonality in the rainfall, with about 80% of the average annual rainfall is available only during four rainy months (June-August). Similarly, average monthly maximum temperature (Tmax) in the EER watershed varies from 22.0 • C in January to 35.9 • C in June, whereas average monthly minimum temperature (Tmin) varies from 7.7 • C in January to 25.4 • C in August.  Notes: SI, station index; SName, Station Name; Lat, latitude; N, northing; Lon, longitude; E, easting; Elev., Elevation; masl, meters above mean sea level; Q, discharge; P, precipitation; SD, standard deviation with units of m 3 /s for discharge and mm for precipitation; CV, coefficient of variation; Mean, long-term average annual value with units of m 3  About 843.5 thousand people, with 51.01% female, are residing within the EER watershed. The population density varies across Palikas, with higher population density in the two large urban centers, such as Hetauda Sub-Metropolitan city and nearby areas, and Bharatpur Metropolitan City (Figure 3). Hetauda and Bharatpur accounts for 44.7% of the total population in the EER watershed.  About 843.5 thousand people, with 51.01% female, are residing within the EER watershed. The population density varies across Palikas, with higher population density in the two large urban centers, such as Hetauda Sub-Metropolitan city and nearby areas, and Bharatpur Metropolitan City ( Figure 3). Hetauda and Bharatpur accounts for 44.7% of the total population in the EER watershed. About 843.5 thousand people, with 51.01% female, are residing within the EER watershed. The population density varies across Palikas, with higher population density in the two large urban centers, such as Hetauda Sub-Metropolitan city and nearby areas, and Bharatpur Metropolitan City (Figure 3). Hetauda and Bharatpur accounts for 44.7% of the total population in the EER watershed.

Methods and Data
The study followed the methodological framework depicted in Figure 4, and the steps are detailed as a flow chart in Figure 5. Daily rainfall and streamflow at eight and three stations, respectively, are considered in the analysis. Key aspects of methodology, as shown in Figure 4, are described in the following subsections.

Methods and Data
The study followed the methodological framework depicted in Figure 4, and the steps are detailed as a flow chart in Figure 5. Daily rainfall and streamflow at eight and three stations, respectively, are considered in the analysis. Key aspects of methodology, as shown in Figure 4, are described in the following subsections.

Data Collection, Screening and Pre-processing
Observed daily time series or streamflow at three stations and rainfall at eight stations were collected as secondary data (Table 2) from the Department of Hydrology and Meteorology (DHM), the government of Nepal. Data quality were assessed in terms of missing data and their distribution/concentration, and the

Data Collection, Screening and Pre-Processing
Observed daily time series or streamflow at three stations and rainfall at eight stations were collected as secondary data (Table 2) from the Department of Hydrology and Meteorology (DHM), the government of Nepal. Data quality were assessed in terms of missing data and their distribution/ concentration, and the reasonableness of recorded values. Data reading, hydrograph/hyetographs for different temporal scales, plot of mass curves, and comparison of means were used as an approach for assessing data quality. Finally, quality controlled data set was prepared for further analysis. Details of hydro-meteorological stations, data availability, and their characteristics, are shown in Tables 1 and 2. In addition to the observed hydrological and rainfall data, population and land use/cover were also collected, as mentioned in Table 2. Population data were collected from Village Development Committee (VDC) (or Palika) and then aggregated for the watershed level to estimate the basin-average population.
Annual time-series of streamflow and basin-average rainfall were generated based on daily time-series. Thiessen Polygon method was adopted for generating basin-average rainfall. Basic statistics, such as mean, median, range, quartile ranges, and coefficient of variation were calculated and analyzed for both streamflow and rainfall time-series to investigate heterogeneity in rainfall and streamflow behavior.

Dealing with Missing Data
The status of missing data in hydrological and precipitation time series is indicated in Table 2. Missing discharge data were filled using 'imputeTS' package in R [25]. One or combination of four methods-Kalman Smoothing, ARIMA, seasonal decomposition, and linear interpolation-depending upon their performance, were used for filling missing data for each of the three series. The method that resulted in positive discharge for the station of interest was used-therefore, methods varied as per station. For instance, at station 470, ARIMA method yielded negative discharges. Seasonal decomposition gave similar results but with few negative values on low flows month, whereas linear was not good in filling 2006 data. Hence, seasonal decomposition was chosen, but negative values were set to value of the preceding month. At station 465, except linear interpolation all other yield negative results. For station 460, ARIMA approach performed well over other methods. In the case of precipitation data, missing daily data were imputed using Inverse Weighted Distance (IDW) interpolation at daily scale with a power factor of 2. over the natural changes [1,22,26,27]. Basically, the process involves detection of an abrupt change point from an analysis of the statistical characteristics of a long-term hydrological series, and then further analysis like quantification of climatic and anthropogenic influences to streamflow is carried out for before and after change point series. Statistical tests, such as Pettitt's test and standard normal homogeneity (SNH) tests, are generally carried out to detect change point. Pettitt test has the advantage of being non-parametric where the assumption of the prior distribution of data is relaxed [13]. SNH test assumes a normal distribution of data [28]. Wu et al. [22] carried out Pettitt's test to detect an abrupt change point for observed runoff from the Yanhe River basin in China during the period 1972-2011, where after 1996, climate change played a dominant role in the decline in the runoff from the basin. Likewise, a study by Sharma et al. [1] also used Pettitt's test, and attributed the decline in streamflow in Tapi River Basin, India to anthropogenic activities, such as changes in the land use patterns at different points of time in different tributaries of Tapi River. Likewise, SNH test has been applied to test the artificial and natural homogeneity of the average annual discharge for eight hydrological stations in the Kupa River Basin, between Slovenia and Croatia [29]. It has also been used to carry a homogeneity analysis of precipitation in Iran [30]. Due to successful application of abrupt change point method in various studies as mentioned, in this study we adopted Pettitt's test together with SNH test [13,14,28] to detect abrupt change (or shift) in the mean of annual runoff, precipitation and temperature time series Both of these methods have been applied in order to complement each other and reinforce the analysis. The significance of the detected change point is determined using a Mann-Whitney U test [31]. Details of the method are provided in Appendix A. Data for the period of 1963-2017 was used. In the case of existence of significant change point, entire time-series was split into two, and the gradual trend was analyzed for two sub-series, before and after the change point. In the case of non-existence of significant change point, gradual shift (or trend) analysis was performed on the entire time series.
Presence of trends in streamflow time series was analyzed using non-parametric Modified Mann-Kendall (M-MK) [17], Spearman's Rho [32] and Sen's slope estimator [33] tests. Mann-Kendall test and Modified Mann-Kendall tests are widely used for the analysis of trends in climatic and hydrological time series [17]. Mann-Kendall test is non-parametric in nature, however it suffers from bias introduced by autocorrelation in nature. The M-MK test addresses this issue by accounting for the presence of autocorrelation in the data in addition to other advantages from the original MK test. Spearman's Rho test, which is also non-parametric is a rank-based approach like M-MK. These tests are, therefore, not affected by distribution nature and are relatively insensitive to outliers. The magnitude of the trend was estimated using Sen's slope estimator. The significance of the detected trend was evaluated using Spearman's Rho test. For this study, like other similar studies on streamflow and climatic series (such as Pirnia et al. [34], Zhu et al. [7], Zhang et al. [35], Zhang et al. [36], Partal and Kahya [37]), the methods mentioned above were used though there are other approaches like segmented regression analysis, which also deals with analyzing trend of time series having change points [38]). Details on these methods are provided in Appendix A.
Furthermore, temporal trends in rainfall time-series were also performed using the same approach as used for streamflow. Spatial variation of trends in annual rainfall and runoff across the study watershed was analyzed in ArcGIS. It was basically done for rainfall data, as there was a good spatial representation of rainfall data compared to runoff data.

Attribution of Streamflow Alterations to Climatic and Anthropogenic Sources
The generic concept for this attribution is that for the catchments with streamflow not subject to any regulation, it can be modelled as a function of climatic variables and catchment characteristics. Furthermore, human factors are assumed to be independent of climatic factors. Therefore, the total change in the streamflow (∆Q) can be expressed as a sum of ∆Q P and ∆Q H [22], where subscripts P and H stands for changes in streamflow, due to rainfall variability and anthropogenic (or human) activities, respectively. We also follow the approach used by Wu et al. [22] for estimating attribution of streamflow changes to climate and anthropogenic changes.
At the local scale, changes in climatic variables are independent to the human behavior shaping catchment characteristic. In the absence of any hydro-regulation, the streamflow response of a catchment can be modelled as a function of climatic variables and catchment characteristics [1]. Here, we have defined the base period (before 1990) and variation period (after 1990). Therefore, the total change in the streamflow over these two periods can be expressed as a sum of changes in streamflow, due to climatic variability and anthropogenic activities [1,22]. They are expressed as: Here, Q total is the total change in the mean annual runoff between the variation period, Q variation , and the baseline period, Q baseline . Q climate and Q human are the changes in the mean annual runoff caused by climatic variability/change and human activities, respectively. µ climate and µ human are the contributions from climate variability/change and anthropogenic factors, respectively, on the runoff variation. Methods like empirical statistics (double mass curve) and elasticity-based can be applied in estimating Q climate and Q human .
In this study, we also applied statistical approaches, such as climate elasticity [20] and double mass curve [21,22]. Their details are provided in Appendix A. After attribution to the streamflow alterations, we further discussed the linkage of attribution to changes in land use/cover and population in the study watershed.

Assessing Socio-Environmental Implications
Streamflow alterations and associated changes in water availability are linked to various aspects of society and environment; therefore, changes in streamflow is expected to have implications in the socio-environmental system. Based on the understanding of the relative contribution of rainfall variability and anthropogenic activities in streamflow changes, necessary secondary data, as well as qualitative information, were collected from various sources, including field studies commissioned during August to October, 2019. The implications were discussed from following three perspectives, which were identified as relevant for the study watersheds after field study: (i) Water and food security; (ii) river health and aquatic biodiversity; and (iii) groundwater abstraction.

Detection of Changes (Both Abrupt and Gradual) in Streamflow
The three hydrological stations cover approximately 32.5% of the 3202 km 2 large EER watershed. However, we estimated changes in streamflow in the EER watershed on the daily hydrological records at those three stations, based on the best available data. For detecting the possibility of abrupt change(s) in the average annual streamflow and precipitation series, the Pettitt and Standard Normal Homogeneity (SNH) tests were applied for selected hydrological and meteorological stations. The time period of analysis, as well as results, are tabulated in Table 3, and time series are plotted in Figure 6.
Results of change point analysis showed that the majority of hydrological and precipitation stations do not exhibit abrupt changes. Statistically significant change point, at 5% level of significance, is observed only at Rajaiya hydrological station in the Rapti river (station 460) from the Pettitt test with data after 1974. Remaining two hydrological stations did not show significant change points. Likewise, these stations show non-uniformity in probable change points ranging from 1974 to 2011 (Table 3), which is very crude to consider any specific time period for change point. In the case of precipitation stations, only Chisapani Gadhi (station 904) displayed statistically significant change point, whereas the change points from other stations were statistically insignificant (Table 3). Like discharge data, they also show diversity in the occurrence of probable change points, but located after 1988. Unlike streamflow and precipitation, all temperature stations displayed significant change points during the late 1980s and early 1990s (Table 3).
For temperature signals too, these change points identified by both Pettitt's and SNH tests are spread over a period of 1982 to 2004 (Table 3). However, it is noticeable that in majority cases, these changes are between 1982 and 1993. We take this period as a transition phase for changes in temperature for the study area. This spread of change points over a period of time have also been reported in papers like Jaiswal et al. [39] and Zarenistanak et al. [40]. Furthermore, the study area experienced a major socio-political change in 1990 that established democracy in the country. Given the local knowledge of the rise in the development process around 1990 after the democratic government started to work in full fledge, cities like Hetauda, located in the eastern part of the basin, have expanded with a boost in its economy and developed as a major economic hub after 1990. Therefore, after considering the local knowledge, as well as temperature signals, 1990 was identified as the change point for this study area. Furthermore, pre-1990 has also been designated as base-period for many climate studies and guidelines. For instance, the definition of climate change indices by the Expert Team on Climate Change Detection and Indices (ETCCDI) considers 1960-1990 period as baseline period (guidelines on analysis of extremes by the World Meteorological Organization) [41]. Moreover, methodologically, there are practices to include 'human-designated' approach to defined change point, and thus, the base-period [22]. We used a mixed approach that consists of looking into the hydro-climatic signal (primarily temperature in this case), as well as local stakeholders' knowledge too, using 1990 as the change point. We, therefore, divided the entire time series of above variables into two sub-periods-the baseline period (until 1990) and the variation period (post-1990).
Then we analyzed gradual shifts (or trends) in streamflow and precipitation time series before and after the change point (i.e., 1990) in order to better understand the extent of changes in streamflow and precipitation and their linkages (if any). We analyzed trends using M-MK approach, computed Sen's slope and tested its significance using M-MK and Spearman's rho statistics. Table 4 and Figure 6 shows the results of the trend analysis. Lothar Khola hydrological station (station 470) exhibit a significant decreasing trend in streamflow during 1964-2017 at the rate of 0.073 m 3 /s/year. Although trends before and after 1990 both decrease, they are not statistically significant. In contrast, streamflow at Rajaiya (station 460) shows a significant increasing trend after 1990, at a rate of 0.467 m 3 /s/year, but the trend for the entire series during 1963-2017 decreases (not significant). The increase after 1990 is attributed to diversion of water from Kulekhani watershed to East Rapti during operation of Kulekhani hydropower project, which came into operation in the late 1980s. Furthermore, the Rajaiya station is just downstream of Hetauda municipality, the urban center. Streamflow at Manahari (station 465) show decreasing trends both before and after a990, but they are statistically insignificant.
In the case of precipitation, it is interesting that except stations 919 and 920 during periods

Relative Contributions of Climatic and Anthropogenic Activities to Streamflow Alterations
We partitioned streamflow changes to climatic and human factors using double mass curve (DMC) and climate elasticity methods. The entire time series was divided into baseline (up to the change point, i.e., 1990) and variation (post-1990) periods (Table 5). DMC, a non-parametric method, uses the long-term relationship between precipitation and streamflow, and provide streamflow response to precipitation. However, it does not take the physical processes into consideration. DMC reconstructs the cumulative annual streamflow (and thus, annual streamflow) in variation period using regression parameters from the baseline period, forced by precipitation in variation period. Thus, the difference in observed mean streamflow in variation period and reconstructed streamflow gives the contribution by the human activities. Results of DMC analysis indicated that human activities are the dominant cause of a decrease in streamflow at stations 460 and 470, and an increase in streamflow at station 465 (Table 5). At stations 460 and 470, they account for 82.2% and 100.4% decrease respectively. The excess 0.4% of the volume is added to the streamflow by rainfall variability/ change (i.e., climate change contribution) at station 470. The increase in streamflow at station 465 is unrealistic as there are no notable human activities and/or water inflow from other systems to the station 465 watershed were identified during the field study. Further diagnosis revealed that observed streamflow at the station 465 for the 2002 and 2003 are 70.83 m 3 /s and 73.71 m 3 /s, respectively, which are 2.7 times higher than the long-term average in the same watershed. Since we could not get an adequate basis to drop data of those two years, it influenced overall trends. Therefore, the results of station-465 should be used cautiously. In overall, results from the empirical statistical method, DMC, reveals that human activities have played a leading role in altering streamflow after the 1990s. We also assessed the impacts of human activities and climate change on streamflow using Budyko hypothesis, an elasticity-based method. Table 6 shows for three sub-watersheds in the EER watershed, the climate elasticity for runoff (i.e., the precipitation elasticity for runoff (ε P )) and the potential evapotranspiration elasticity for runoff (ε ET0 ) were estimated from eight water-balance models, two based on nonparametric methods, and six based on the Budyko hypothesis. As shown in Table 6, the ε P ranged from 0.267 to 2.075 for the East Rapti sub-watershed (Inde = 460), from 0.821 to 2.132 for Mahahari sub-watershed (station 465) and from 1.077 to 2.171 for Lothar Khola sub-watershed. Likewise, the ε ET0 ranged from −1.075 to −0.673 for East Rapti, −1.132 to −0.252 for Mahahari and from −1.904 to −0.806 for Lothar Khola. These results, for the East Rapti sub-watershed, indicate that a 1% change in precipitation will result in a 0.267% to 2.075% change in the runoff, a 1% increase in potential evapotranspiration will lead to a 1.075% to 0.673% decrease in the runoff. There were differences in the estimates of elasticity coefficients among the eight methods. Studies, such as Sankarasubramanian et al. [42] and Zheng et al. [20], estimate that the precipitation elasticity for runoff (ε P ) is lower in comparison to the estimations from the Budyko hypothesis. However, evapotranspiration elasticity for runoff (ε ET0 ) estimates for non-parametric approaches have similar values with estimates from the Budyko hypothesis. It is to be noted that aridity index for East Rapti, Manahari, and Lothar Khola sub-watersheds are 0.673, 0.714, and 0.744, respectively, implying that basin is humid. The influence of climate change and human activities on streamflow alteration as calculated by different climate elasticity methods are shown in Table 6. For the East Rapti sub-watershed, non-parametric methods and the Budyko hypothesis showed two contrasting results. Non-parametric methods showed that climate change-induced runoff declined by 57.6 to 82.3 mm, which accounts to 66.1 to 94.5% of total change, and we notice that evapotranspiration played a major role in this decline. In contrast, the Budyko hypothesis attributed the climate factor for the addition of runoff by 38 mm to 49.3 mm, while human factor reduced runoff by 125.1 to 136.4 mm; thus, with a net effect of reduction of runoff. From the field study, it is clearly seen the anthropogenic activities are the major reason for alterations in streamflow. In the Manahari river sub-basin, runoff is reduced by 29.4 to 93.8 mm, due to climate effects, both precipitation and evapotranspiration leading to a reduction in the runoff. In the Lothar Khola sub-watershed, non-parametric approaches yield similar magnitude of decrease in the runoff (45.5% to 48% by climate factor and 54.5% to 52% by an anthropogenic factor) in the basin. In contrast, Budyko hypothesis-based method showed that the influence of climate change is limited to 15.9% to 24.6 % only, the rest (75.4% to 84.1%) is due to the human factor.
In a nutshell, human activities are playing a dominant role in altering, more specifically, decreasing streamflow after 1990 in the EER watershed. Climate change is also contributing to alterations in streamflow, but less dominantly compared to anthropogenic activities, and contribution of evapotranspiration is significant in climate change-induced alterations.

Socio-Environmental Implications of the Streamflow Changes
Alterations in streamflow have implications in various sectors. Growing population (Appendix C), urbanization (eight out of 15 Palikas in EER watershed are Municipalities), land use/cover changes (Appendix D) and climate change/variability, among others, are contributing to alterations in streamflow. As a result, there are implications on water availability and food security, river health and aquatic biodiversity, and groundwater abstraction, among others.

The Implication to Water and Food Security
The EER watershed is home for nearly 765,000 populations [50] distributed across 15 Palikas with varying densities ( Figure 5). In Chitwan district, which covers nearly 55% of the EER watershed area, the population has increased by 123% during 1981-2011 (Appendix C). Furthermore, feeding to the growing population needs more food production, and most of the irrigation water for food production is withdrawn from rivers within the EER watershed.
As per the National Irrigation Master Plan [51], Chitwan district currently has 20,643 ha of irrigated land through 53 irrigation projects, which is nearly half of total irrigable areas of 42,000 ha. Similarly, in Makwanpur district, out of 23,500 ha of irrigable areas, only 2406 ha are covered by irrigation through 33 irrigation projects. Therefore, with a future emphasis of all the three-tier governments on intensifying agricultural activities to feed a growing population, more irrigation coverage is expected in near to mid-future. With more increase in irrigated areas and associated water use coupled with altered streamflow, crop production and food security are likely to get affected unless climate-resilient agricultural practices are put in place. The alterations in streamflow either from climatic or anthropogenic origin can introduce new challenges for watershed-level management of water resources, and more specifically, transboundary water management in the EER watershed, which are shared by 15 local governments in the new federal governance structure of Nepal.
From the analyses presented in Section 4.1, it is clearly seen that the EER watershed exhibits a decreasing trend in streamflow post-1990. The persistence of such trends in the near future would potentially escalate the water crisis and conflicts in the watershed. Thus, rapidly increasing population and declining freshwater availability may affect the food grain production, which would likely manifest food shortages in the area.
Agricultural areas in the Chitwan and Makwanpur districts in the EER watershed have increased from 28.5% to 29.7%, and from 19.7% to 22.2%, respectively, during 2000 to 2010 (Appendix D), which have manifested an increase in irrigation water requirements. The annual production of main crops (i.e., paddy, wheat, maize, and millet) have increased in general in both the districts that share the EER watershed (Table 7). The rising trend in the production of water-intensive crops, such as paddy, is further posing stress on available water resources in the watershed. It would further be aggregated by land use/cover change in the watershed. Though there are no data available on land use/cover change trends, as well as future projections to quantify impacts, a general tren is the decrease in forest areas as in other regions of Chure belt in Nepal, decrease in open areas, and increase in built-up areas in and around the urban centers. This phenomenon has socio-economic implications in terms of increase in intensity and frequency of flash floods, deterioration of water quality, and decrease in recharge to groundwater aquifers, among others; all of which have implications on water (quantity, quality, and timing). Projecting future land use/cover and quantifying the extent of impacts on the aforementioned aspects of water could be considered as a next step for furthering this research.
Alterations in streamflow may affect the production of paddy and other crops and therefore have implications in food security. Interactions with locals in the watershed further confirm that changes in agricultural areas are already a reality. Due to declining water availability, farmers are resorting to growing crops like maize that require less water than growing water-intensive crops or leaving the land fallow altogether. The impact is further exacerbated, due to the fact that many farmers practice subsistence agriculture, and few are sharecroppers.
On the other hand, the alarming rise in population (e.g., a 123% increase during 1981-2011 in Chitwan district) has manifested an increase in water and food demands. Availability of edible food (defined here as rice, maize, wheat, barley and buckwheat) has decreased from 87,662 metric tons (MT) in 2011/12 to 67,448 MT in 2014/15 in Makwanpur district [52,53]. Similar trends are observed in Chitwan district as well; a reduction from 86,581 MT to 22,875 MT for the same period. As the population is growing at the rate of 2.3% in the area, the per capita food availability (kg/yr) during the period is also certainly decreasing. Though the quantitative impact of water stress conditions on per capita food availability could not be ascertained (due to lack of data), it is highly likely. If it persists for longer period, it may lead to food insecurity in the watershed. To ensure food security and avert a crisis in the future, strategies for water demand management (e.g., efficient irrigation methods, develop and practice climate-adaptable crop varieties, etc.), as well as supply side management (e.g., practice rain water harvesting, implement water conservation measures, such as efficient recycle and reuse of water, educate and raise awareness of users on water conservation, etc.), need to be devised. Moreover, implementation structures need to be put in place, and monitoring and evaluation of effective implementation must be carried out.

The Implication to River Health and Aquatic Biodiversity
Healthy rivers have the potential to provide surrounding communities with a variety of benefits. Streamflow regime plays a key role in shaping the structure and health of water environments [57]. Alteration in the natural flow regime, due to interventions on water and land resources, can impact adversely on riverine ecology, morphology, riparian rights of the downstream users, and downstream water environments [1,58]. Land use/cover change in the form of deforestation may contribute to the degradation of landscape, soil erosion and sedimentation, which have adverse implications to river health and aquatic biodiversity. Furthermore, change in the river regime as a result of land use/cover change may affect the growth and reproduction of some flora and fauna in the riverine ecosystem. Other aspects of land use/cover change, such as urbanization, may contribute to flash floods and the subsequent change in flow regime, as well as the discharge of polluted water directly to the freshwater system, which can disrupt natural environments and degrade ecosystems [59,60]. This phenomenon can ultimately cause irreversible consequences for both human and ecosystems [61]. Therefore, alterations in water availability, but increasing water demands in the EER watershed will increase competitions between human use and environmental needs, which further affects adversely to river health and aquatic biodiversity.
The decreasing streamflow in the study area has deteriorated capacity of rivers to dilute pollutants, due to excessive use of chemical fertilizers and pesticides in agricultural lands and industrial pollutions generated from at least 134 various types of industries [62] in the Hetauda Industrial District. Therefore, river systems are more polluted than before, even though there are no studies to quantify the extent of degradation in river health. As more industrial districts (e.g., Shaktikhor in Chitwan and Mayurdhap in Makwanpur [63], etc.) are planned in the EER watershed, if industrial wastewater is not pre-treated before disposing to the river, it will certainly affect negatively to aquatic biodiversity. Washing of vehicles in the river, which is widespread in the East Rapti River, and associated mixing of automobile chemicals will have impacts on aquatic flora and fauna, which are not yet all studied. In addition, riverbed mining, which is widespread in the EER watershed, is also affecting adversely to river ecology, river health, and aquatic biodiversity. There is also a hypothesis that recent cases of deaths of over 40 rhinos in the Chitwan National Park, as well as the decline in fish production in Markhu, Kulekhani over the decade, could potentially be linked to the deteriorated water quality of the rivers as one of the causes, though there is no proof so far.
In a nutshell, although there are no scientific studies to quantify them, anthropogenic activities (such as industrialization, intensified agriculture, and urbanization) are degrading river health, and affecting adversely to aquatic biodiversity in the EER watershed. Moreover, they may also accelerate the eutrophication in water bodies through an increase in the rate of nutrient inputs draining into the streams. Alteration in streamflow coupled with extensive eutrophication, may affect adversely the already degraded self-purification capacity of the river system, degrades water quality, and pose a threat to the aquatic flora and fauna. Therefore, a cooperation framework among local riparian governments for managing the shared EER watershed could be the potential pathway for the prosperity of the local governments, as well as of the watershed. The framework, from the perspective of maintaining river health and preserving aquatic biodiversity, could potentially be operationalized by introducing the concept of Environmental Flow (E-flows) and ensuring commitments of all the local governments to comply with that. As new governments are ready to frame their development strategies, it is the right time to introduce this intervention for ensuring healthy rivers, maintenance of aquatic biodiversity, and sustaining human life for a long run.

Implications on Groundwater Abstraction
It is estimated that population growth in the EER watershed has increased domestic water demand alone by 140%, during 2011-2019, which is projected to increase further by 2050. Furthermore, need for food, energy and other resources for the increased population has resulted in an increase in agricultural areas over the past decades (Appendix D). The trend is expected to increase further in future, in order to cater for more food for growing population. This will have implications in water demand. As identified in the field study carried out during October 2019, average duty in the study area is 2 L per second (lps) per hectare in the study area. It means that per hectare increase in the irrigated area increases water demand by 2 lps. In this context, given relatively flat topography of the EER watershed (about 33% of the EER watershed lying in Tarai, i.e., at the elevation below 300 m) and availability of groundwater resources, pressure on the precious groundwater resources are likely to increase. Domestic and agricultural activities in various locations within the EER watershed are critically dependent on groundwater resources. About 46% of the water used by households in the watershed for drinking purpose is abstracted through groundwater sources [50]. Potentially declining groundwater levels, coupled with alterations of streamflow, is expected to have an impact on domestic uses, as well as food security. If the groundwater abstraction is not regulated properly, groundwater resources are likely to deplete, though there are no scientific estimates on groundwater recharge, the current level of groundwater abstraction, and potential impacts of an increase in water demand. For example, Hetauda Sub-Metropolitan City (HSMC) area alone has nearly 25 tubewells with depths and discharge varying in a range of 76-105 m and 5-32 lps, respectively, are drilled during 2004-2009 (Source: Personal communication with Groundwater Resources Development Board, Chitwan in September 2019) to abstract groundwater for irrigation. Furthermore, the Irrigation Master Plan [51] has included Chitwan as one of the priority districts for groundwater projects. These trends are expected to continue to cater to water demands in the EER watershed for domestic, irrigation, industrial, and other purposes.
Urban pockets in the EER watershed, such as HSMC, Bharatpur Municipality, and others, are expected to abstract more groundwater resources while moving farther in the future, thus exerting more pressure on the precious resource Population in the HSMC areas grew at the rate of 2.6%, but urban areas increased by four-fold during 1995-2011 [64], which is continuing to increase after it became a sub-metropolitan city with the recent change in the governance structure of the country to federal. Water-intensive lifestyle in urban areas, which are getting supply mostly from groundwater, is already adding more pressure on groundwater resources through increased abstraction. Groundwater production by Hetauda Water Supply Management Board (HWSMB) has increased by 25% during 2016-2018 to supply water for the HSMC dwellers. Furthermore, groundwater abstraction as the major source of irrigation supply in the vast majority of agriculture areas is expected to continue for, at least, the foreseeable future. In the context of further alterations in surface water, groundwater abstraction would further be accelerated as a safe and reliable source of water supply for various uses in the parts of the watershed where feasible. On the other hand, land use/cover change in the form of urbanization has decrease open and cultivation areas in the HSMC area, which recharge the groundwater system, by at least 7% during 1995-2011 [64], which affects adversely to recharge and then to groundwater availability. Though there is no detailed mapping of groundwater potential and aquifer depths in the EEER watershed, groundwater aquifer in the flat southern part of the watershed is expected to be a single unconfined aquifer with about 200 m thickness in HSMC areas with variation in depth across the watershed. The static groundwater level in the HSMC area is at a depth of 10-58 m below the ground surface, and groundwater in the area is being abstracted from nearly 25 wells to cater for nearly 86% of the water demand in the HSMC (source: Authors analysis based on secondary information collected during a field visit in 2019). All these scenarios reflect continued increase groundwater abstraction in the future.
From interaction with stakeholders during a field visit in mid-2019, it was noted that groundwater levels in a couple of wells were depleted over the years. As new governments are keen to do something new to contribute to the prosperity of their constituency, it is likely to put more pressure on groundwater resources, but it is not well understood and represented to inform decision-making. In overall, increase in industrial and urbanization activities and continued need of water for agricultural intensification to fuel food security is expected to impact adversely to groundwater levels in parts of the EER watershed. Furthermore, lowering groundwater levels will likely reduce groundwater contribution to natural streams, which will further contribute to declining streamflow across the basin. Therefore, a baseline study for stock-taking of the current state of groundwater availability, withdrawal, recharge, and estimates on the allowable level of groundwater abstraction would be helpful for strategically using groundwater resources, without conflict with adjoining local governments, for the prosperity of the EER watershed.

Conclusions
This study, regarding the Extended East Rapti (EER) watershed in central-southern Nepal, evaluated trends in rainfall and streamflow, estimated relative contributions of rainfall variability and anthropogenic activities towards streamflow changes, and discussed potential socio-environmental implications of the streamflow alterations. Key conclusions from the analysis are: • Two distinct trends in hydro-climatic time series (temperature) exist before and after 1990. Therefore, 1990 can be considered as the change point for the abrupt change in the EER watershed. Streamflow alterations are having implications to water and food security; river health and aquatic biodiversity; and groundwater abstraction. However, a separate study is required to quantify precisely the extent of impacts.
To ensure water and food security, river health, aquatic biodiversity, and to be able sustainably abstract groundwater, it is recommended to devise strategies for water use efficiency through both supply-and demand-side management of water. It is also crucial to develop a water co-operation framework among the local governments, which share the same water resources, for strategic and sustainable use the water resources, and river health preservation.

Appendix A.3. Modified Mann-Kendall (M-MK) Test
Modified Mann-Kendall (M-MK) test [17] for the trend is a modified version of non-parametric rank-based Mann-Kendall (MK) test [66,67]. Original MK does not require the data to be normally distributed, robust to influence of extremes and has low sensitivity to abrupt breaks due to inhomogeneous time series [39]. However, MK test is often biased in the presence of the autocorrelation, i.e., when the time series is influenced by previous observations [17]. In order to account for the presence of autocorrelation in data, Hamad and Rao [17] proposed variance correction in original MK.
In the MK test, the null hypothesis H0 is that data are independent and randomly placed with no serial correlation structure among the observations. The test statistics are given as: where n is the total length of the data, x i and x j are two generic sequential data values.
The test statistic S has mean zero, and the variance is given by: where n is the length of time series, and t is the extent of any given tie, and Σt denotes the summation over all tie number of values. In cases where the sample size n > 10, the standard normal variate z is computed using Douglas et al. [68] as: The positive value of z statistic represents a rising trend and negative value indicate a declining trend in series. The null hypothesis is accepted if |z| ≤ zα/2 at α level of significance in a two-sided trend of the test.
Hamad and Rao [17] proposed the following changes in Var(S) with correction needed due to the autocorrelation in the data, which is as follows: where Var(S) mod is modified variance, and n n * S is a correction due to autocorrelation in data. n n * S is computed as: where n is the actual number of observations and ρ S (i) is the autocorrelation function of the ranks of the observations. For the computation of ρ S (i) autocorrelation should be significant.
Thus, the contribution to changes in the runoff from human activities can be calculated as expressed in Equation (A3): Furthermore, the contribution to the runoff from climate change can be calculated using Equation (A4): Here, Q reconstruct and Q variation , respectively, represent the average of the reconstructed runoff and the observed runoff during the variation period Appendix A.7. Elasticity-Based Approaches In an unregulated catchment, the runoff from the catchment is the function of precipitation, evapotranspiration and catchment characteristics. In the elasticity approach, assuming that the changes in precipitation and potential evapotranspiration are the main relevant climatic changes, we can write as in Equation (A5): where, ε p and ε ET0 are precipitation elasticity for runoff and the potential evapotranspiration elasticity for runoff, respectively. In general, we can define the climate elasticity of runoff (ε) is defined as the ratio of the runoff variation rate to the variation rate of a certain climate factor (precipitation or potential evapotranspiration in this study). The precipitation elasticity for runoff (ε P ) and the potential evapotranspiration elasticity for runoff (ε ET0 ) can be estimated using various ways. In this study, we used the two nonparametric approaches and six water balance-based approaches (from the Budyko hypothesis) to estimate the precipitation elasticity for runoff (ε P ) and the potential evapotranspiration elasticity for runoff (ε ET0 ).
Appendix A.7.1. Non-Parametric Approach: Sankarasubramanian et al. [42] proposed a nonparametric method to estimate climate elasticity directly from the data, which is shown as Equation (A6): where Q i and X i are the changes in annual runoff and the relevant climate variable (e.g., precipitation, potential evapotranspiration), respectively. Q i , X i are the annual runoff and climate-variable values, and Q, X are the average annual runoff and climate-variable values. Sankarasubramanian et al. [42] used the median of ε i as the estimated value for elasticity. Zheng et al. [20] developed a method of estimation of elasticity using the least squares estimator as in Equation (A7): Appendix A.7.2. Budyko-Based Methods: Climate elasticity can be often obtained from the long-term water balance of the catchment. Water balance of a catchment is given by Equation (A8): where P, E, Q and ∆S are precipitation, actual evapotranspiration, runoff and change in storage of a catchment. When a long-term period is considered, the change in storage is assumed to be zero. Therefore, a ratio of Q and P is expressed as Equation (A9), According to Budyko [45,71], the actual evapotranspiration E is a function of the dryness index or aridity index, φ = ET0/P. Now, from the definition of elasticity and above equations, we can calculate the precipitation elasticity coefficient and potential evapotranspiration elasticity coefficient for runoff as Equation (A10) and Equation (A11): F(φ ) represents the derived function (derivative) of F(φ). Estimation of F(φ) are given by six approaches are given in Table A1. The parameters m in the Fu et al. [48], and ω in the Zhang et al. [49] function are functions of the vegetation, soil, topography, and climate [7].

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Pike [46]; Turc [47] ( ) = 1− −2 Fu et al. [48] ( ) = 1 + − (1 + ) 1/ Zhang et al. [49] ( ) = 1+ (1+ + 1 ) The parameters m in the Fu et al. [48], and ω in the Zhang et al. [49] function are functions of the vegetation, soil, topography, and climate [7].  Measured maximum and minimum temperature series in three stations in the study area during their observational length (1968-2017) show significant increasing trend at a rate between 0.02 • C and 0.09 • C per year. In contrast to the observation for the entire period, in the case of maximum temperature, we can observe that stations 902 and 906 show a decreasing trend after 1990 (NS) at a rate of 0.005 • C/year and 0.016 • C/year, respectively. Station 905 showed an increasing trend (S) at a rate of 0.09 • C per year. All the stations showed an increase in minimum temperature after 1990, with a maximum increase of 0.093 • C per year for station 905. Station 905 also shows the highest rate of increase in maximum temperature at a rate of 0.09 • C per year. Stations 902 and 906 showed an increasing rate at 0.003 • C and 0.025 • C per year, respectively.
The trend of increase in daily minimum temperature is an indication of growing warmness during the night in the study area. This will relate to an increase in evapotranspiration in the basin. From trend analysis precipitation, we observe that six out of eight stations data show a decrease in annual precipitation (though only one is statistically significant). This follows a decrease in mean annual runoff in stations 465 (Manahari River) and 470 (Lothar Khola) after 1990. Significant increase in the runoff after 1990 in station 460 (East Rapti) can be attributed to two reasons; first reason can be the increase in rainfall in station 905 (though data is limited to say definitely) and second reason can be the addition of discharge from KuleKhani reservoir hydropower project. The influence of climatic factors and human factors will be further analyzed in the following sections.