Pan Evaporative Changes in Trans-boundary Godavari River basin, India

Pan evaporative changes are one of the key components of water resources management of a basin under changing climate and anthropogenic-induced warming. This study was undertaken for trans-boundary Godavari River (India) to identify trends through the Mann-Kendall (MK) test after removing the effect of significant lag-1 serial correlation from the climatic time-series by pre-whitening in pan evaporation (Epan) and in the probable causative meteorological parameters responsible for evaporative climatic changes in a large basin. Further, the Pettitt’s test was applied on Epan time series for estimating the change point year of Epan to find out the effective year when the change in pattern started reflecting in the time-series. At seasonal (monthly) time scales, statistically significant decreasing trends in Epan were witnessed in pre-monsoon season (in the months of March, April and May) over all the seven sites of the Godavari basin. Four sites witnessed statistically significant increasing trends in Tmin (Tmax) in July (December) and in monsoon (post monsoon) season in the basin. Statistically significant decreasing (increasing) trends in wind speed (relative humidity) in pre-monsoon and in month of March at these seven sites support the observed decline in the evaporative demand in the basin leading to possible enhancement in the total yield of the basin. Results of stepwise regression analysis showed that wind speed followed by relative humidity was found to be two main causative parameters of the observed decline in the Epan under the warmer environments in the basin. Pettitt’s test shows year 1991-1992 to be the probable year of change in the Epan in the Godavari river basin.


Introduction
Evaporation, one of the main components of hydrologic cycle, which plays a vital role in agricultural and hydro-meteorological studies, water resources management and irrigation scheduling (Gundekar et al., 2008). Evaporation is influenced by a large number of meteorological factors, such as, air temperature, relative humidity, sunshine duration and wind speed. The process of evaporation is a complex function of several parameters and change in one parameter can influence other parameter(s). Changes in air temperature can modify the saturation vapor pressure, which in turn may alter the evaporation rate as well .
The global air temperature has increased by 0.6°C in last century due to anthropogenic factors, such as population growth, deforestation, changes in land use and increase in atmospheric (2014) conducted change detection study on climatic parameter over Raipur district of India and found significant rising trend in summer temperature with rising evaporation process attributed to increased wind speed and temperature. Krishan et al. (2017) analysed the trends in rainfall and dry/wet years over a canal command area of Northern India. Study found a significant decreasing trend in annual and monsoon rainfall. Hadi and Tombul (2017) reported significant increasing trend by using the Mann-Kendall test in temperature with 0.88 °C/century whereas precipitation showed insignificant increasing trends over Turkey. Jhajharia (2012) reported rise in air temperature over a southern peninsular river basin of India using the temperature data of 35 sites located indifferent sub-basins of the Godavari River.
The reported temperature rise in the Godavari convinced to establish if evaporation may have increased in the warmer climates affecting water availability in the Godavari basin. Thus, Godavari River is selected with the objective of studying trends in pan evaporation (Epan) through the Mann-Kendall (MK) test in annual, seasonal and monthly time scales in the current study. The procedure of pre-whitening was applied to remove the effect of significant lag-1 serial correlation, if any, from original time-series of the Epan. Trends were also identified in temperature, wind speed, relative humidity and sunshine duration by using the MK test. Further, the stepwise 3 regression method is used to search for the principal climatic variables associated with Epan and possibly explain the underlying mechanisms of observed pan evaporative changes in the Godavari basin.

Details of study and meteorological data
Godavari River is the largest river of peninsular India and is held in reverence as "Dakshin Ganga" (Ganges of South). Several holy places are located on the banks of the river at Nasik and  Godavari River is purely rainfed and its main tributaries include Pravara, Purna, Manjra, Maner, Penganga, Pranhita, Indravati, Sabari, etc. The river carries enormous quantities of water during monsoon. The basin consists of large undulating plains separated by low flat topped hill ranges and the main soil types found are black soils, red soils, lateritic soils, alluvium, saline and alkaline soils (CWC 1987(CWC , 1999. The average annual rainfall of the basin is about 1132 mm, and the mean surface temperature ranges from 14.5°C to 35.5°C in different parts of the Godavari basin (Jhajharia et al., 2014). The monthly data of Epan were obtained from the India Meteorological Department (IMD), Pune (Maharashtra), for different periods, especially from 1969-2007, at seven stations, namely, Aurangabad, Betul, Hyderabad, Jagdalpur, Nagpur, Yeotmal and Ramagundam located in different sub-basins of the Godavari. The details and location of selected sites are given in Table 1 and Fig. 1, respectively. The monthly data set were 4 used to obtain the annual and seasonal values of pan evaporation for these sites of the basin. The monthly and annual average data of total Epan of all the stations of the basin are given in Table 2.
The total average annual Epan values vary in the range of 1400.80 mm to 2129.70 mm in the Godavari basin.
Location of Fig. 1 Location of Table 1 Location of Table 2 2

.2 Trend Analysis
In the present study, the MK test was used for detecting trends in Epan and other climatic parameters as non-parametric MK test is more suitable for non-normally distributed and censored then the null hypothesis of no trend can be accepted at significance level of α. Otherwise, the null hypothesis can be rejected and alternative hypothesis can be accepted at significance level of α.

Change Point Detection
The Pettitt (1979) test is a non-parametric change detection test, which is used finding the probable year of change in the pattern of the recorded climatic time series. It detects change in the mean of a time series and the year when the change starts reflecting in the series. The nonparametric test statistics for this test may be described as follows: (4) The test statistic K and the confidence level (ρ) for the sample length (n) may be described as following: When ρ is smaller than the specific confidence level, the null hypothesis is rejected. The approximate significance probability (p) for a change-point is defined as given below: 3. Results and discussion 6 Trends in Epan and other climatic variables, i.e., temperature, bright sunshine duration, wind speed and relative humidity were obtained through the Mann-Kendall test for different durations: annual; seasonal: winter, pre-monsoon, monsoon and post-monsoon; and monthly: January to December. The results are discussed below as follows.

Analysis of temporal trends in Epan
The records of Epan are measured through the USWB Class A Pan Evaporimeter at seven sites The trends in Epan at seven sites in monthly, annual and seasonal time scales were identified through the non-parametric MK test. Table 3 shows the Z statistics values obtained through the MK test for identifying trends in Epan over seven sites located in Godavari basin in different time scales. It can be inferred from Table 3  annual time scale, i.e., all the sites located in the basin witnessed decreasing trends in winter, premonsoon and monsoon seasons. However, at least six sites (five sites each) witnessed statistically significant decreasing trends in Epan at 1% level of significance in pre-monsoon and monsoon (winter and post-monsoon) seasons over Godavari (see Table 3

Trends in Relative Humidity and Temperature
The data of the morning relative humidity (RHmax) and the afternoon RH (RHmin) are usually  Table 6 shows the values of the Z statistic obtained through the MK test for identifying trends in wind speed for the seven sites located in the Godavari basin at monthly, annual and seasonal time Bright sunshine duration data are recorded at two sites (Hyderabad and Nagpur) located in the Godavari River basin on monthly basis since 1969. Table 6 shows the values of the Z statistic

Searching evidence for the existence of evaporation anomaly
IPCC (2007) has stated in its report that the increase in the global temperature is due the increased anthropogenic emission of greenhouse gases. As the temperature is one of the significant factor in driving the evaporation mechanism, increase in temperature would enhance the evaporation process. However evaporation anomalies have been observed around the world where the evaporation rate contradicts the increasing temperature. It is interesting to note that the decreases in Epan in the Godavari basin despite the reported temperature increase can be attributed to the other dominating climatic parameters, i.e., wind speed, humidity, sunshine hours, etc.
Stepwise regression analysis showed that wind speed (RHmax) was found to be the main driving factor, which affected trends in Epan in winter, pre-monsoon and monsoon (post-monsoon) over Godavari basin. On annual time scale, wind speed followed by the RHmin, RHmax and Tmax were found to be the main causative parameters of the observed trends in Epan over Godavari basin. On the monthly time scale, wind speed followed by relative humidity (relative humidity followed by wind speed) was found to be two main causative parameters of the observed trends in Epan during the months of January to June (July to December) over Godavari basin.

Spatial variation in Pan Evaporation and Changes in Climatic Variable
Godavari River is a trans-boundary river which partially covers the States of Central, Eastern and Southern India. Elevation variation in the river basin varies from 1067 m at the origin point to 329 m at the confluence point in Bay of Bengal. Thus, spatial variation map of pan evaporation was prepared for annual and seasonal timescale to find out the seasonal variation in these parameters. Our results may have potential for the adoption of climate-related changes in the Godavari Water Dispute Tribunal (GWDT) award in future by the policy makers as currently the award related to the optimal utilization of Godavari River shared by six states is adopted permanently.

Declaration
Funding: Not applicable.

Conflicts of interest/Competing interests:
There is no conflict of interest.

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Availability of data and material: Authors would like to thank India Meteorological Department for providing the meteorological data.
Code availability: Not applicable.