Evaluation of integrated impacts of climate and land use change on the river flow regime in Wamkurumadzi River, Shire Basin in Malawi

Climate and land use change (CC and LUC hereafter) are interlinked factors that can lead to river flow regime changes, as well as affecting hydrological extremes such as floods and drought. There is now considerable evidence of CC and LUC in many catchments in Malawi but without corresponding evaluations of their impacts on river flow regimes. Therefore, this study assessed how both factors affect the flow regime of Wamkurumadzi River, a key tributary of the major Shire River in southern Malawi. Land use and hydroclimatic data for the basin were first analyzed for spatial–temporal trends in the historical period between the years 1984 and 2015. The Soil and Water Assessment Tool (SWAT) model was then applied with different LUC and CC scenarios in order to assess their sole and combined impacts on the river flow regime. The model was calibrated and validated using the split sample method from the year 1984 to 1999 and from the year 2000 to 2015. Model performance was acceptable according to the selected evaluation criteria, with the Nash–Sutcliffe (NSE) coefficient of 0.78 and coefficient of determination (R) of 0.96 during calibration and NSE of 0.93 and R of 0.98 during validation. Results of the integrated impacts of LUC and CC suggest a slight increase in river discharge of 0.05 m/s for the period between the 1980s and 2000s. During the 1980s–1990s, both CC through rainfall decreased and LUC resulted in decreases in the mean river discharges by 1.58 and 0.37 m/s, respectively. The study also found that CC through increased rainfall in the 1990s–2000s decades saw an overall increase of 1.39 m/s in mean river discharge, while LUC shows the increase of mean river discharge by 0.25 m/s. However, the study observed that reforestation efforts in the basin were greatly responsible for the alteration of the river flow regime in the later period.


INTRODUCTION
Southern Africa is among the regions where the impacts of climate change are projected to be worse than in most other regions of the world due to a web of factors such as frequent occurrence of climate risks (floods and drought), low adaptive capacity, high rates of poverty and overdependence on rainfed agriculture. In addition, the region has a fast-growing population and urbanization rates which exerts considerable pressures on the environment and natural resources including forest cover. In the region's key river basins such as the Zambezi, studies have already established that air temperature and potential evaporation are projected to increase while rainfall is projected to decrease, potentially leading to decreases in river flows and increased reservoir evaporation losses (Hamududu & Killingtveit ) Apart from climate variability, Malawi as a country has a high dependence on cultivated agriculture which can be detrimental to water resources by either leading to an increase or decrease in river flow. In addition, deforestation in Malawi has been rampant due to agricultural activities, human settlement and charcoal making. Consequently, Malawi has experienced considerable changes in the forestry cover from 50% in 1960 to 45% in 1972 and to 41% in 1990 (Environmental Affairs Department (EAD) ).
The forest cover declined from 3.9 million ha in 1990 to Such changes are also normally catchment specific.
Wamkurumadzi River is a major perennial tributary of the Shire River, Malawi's major socio-economic asset. The river has the potential to supply water for irrigation and other domestic activities within the catchment, including potential sites for dam and irrigation schemes development.
Currently, no dam has been constructed and only eight smallholder farmers operated mini-irrigation schemes of sizes ranging between 0.2 and 3.5 ha. According to MacDonald (), the use of Wamkurumadzi River for supplying water within the catchment is currently limited. Most of the people living within the catchment area depend on agriculture and charcoal production for survival which pose some threats to the river due to LUC. On average, Neno District in the catchment produces the largest quantity of charcoal which is supplied to other towns and cities such as Blantyre (Mutimba & Kamoto ). Consequently, the catchment has experienced considerable degradation due to deforestation and other human activities, coupled with climate change which, in turn, is affecting river flows. However, there is no study that has been undertaken to validate the main or joint factors affecting the river flow. Such analysis is important especially in Wamkurumadzi River, as potential dam sites were already identified. Therefore, the general objective of the study is to evaluate the integrated impacts of climate change and land use change on the river flow regime of the Wamkurumadzi River in southern Malawi. Specifically, the study aimed to (1) investigate the nature of temporal changes in land use in the catchment; (2) examine trends in climate in the catchment and (3) analyze the integrated impacts of LUC and CC on river flows.

Description of the study area
Wamkurumadzi River is one of the major perennial tributaries of the Shire River, with its headwaters in the Kirk  shown that large parts of the forest land have been replaced with agricultural areas for crop production and trees have been cut for timber, firewood and charcoal production.

Hydroclimatic data sources and analysis
The study used daily climate (rainfall and temperature) and hydrological data for the stations and record lengths are shown in Table 1 Project at the University of Strathclyde. All the datasets were of relatively good quality but were nevertheless checked for quality control such as outlier detection.
The direction and significance of temporal trends in the hydroclimatic variables (rainfall, temperature and river discharge; Table 1)  The SWAT model divides the catchment into numerous sub-catchments and a total of 29 such sub-catchments were found for the Wamkurumadzi basin. These sub-catchments were further divided into elementary hydrologic response  in this phase as follows: where SW t is the final soil water content (mm); SW 0 is the initial soil water content on day i (mm); t is the time (days); R day is the amount of precipitation on day i (mm); Q surf is the amount of surface runoff on day i (mm); E a is the amount of evapotranspiration on day i (mm); W seep is the amount of water entering the vadose zone from the soil pro- and the ratio of root mean error squared (RMSE) to observations standard deviations (RSR) given as follows:

Evaluation of the impact of LU and CC on streamflow
The approach of one factor at a time was used in order to evaluate the impact of LU and CC on streamflow using the calibrated SWAT model (Table 2) To evaluate the differences in the simulated mean discharges under the various scenarios, the student's t-test was used under the null hypothesis of equal sample means (H 0 : x 1 À x 2 ¼ 0) and alternate hypothesis of unequal sample means ( H 1 : x 1 ≠ x 2 ) calculated as: where x 1 and x 2 are the mean discharges in the baseline and scenario period, respectively, and S 1 and S 2 are the respective standard deviations in the baseline and scenario period.
At 95% confidence level, the null hypothesis is accepted if jtj < 1:96 and rejected otherwise.

LUC between 1989 and 2015
The results of supervised classification of Landsat images from the years 1989, 1999 and 2015 are shown in Figure 3(a)-3(c) and are further summarized in Table 3.
The results show considerable LUC in the Wamkurumadzi catchment area. It can be seen from Figure 3 and Table 3 that agricultural land had covered 30.63% in 1989, but this had decreased to 7.62% in 1999 before increasing to 15.14% in 2015. In addition, the urbanized area increased rapidly between 1989 and 1999, followed by a slight decrease in 2015. Furthermore, the area covered by forest decreased from 24.88% in 1989 to 19.76% in 1999 before increasing slightly to 20.68% in 2015. Rapid population growth in the area is among the factors that have led to increased demand for agricultural land and residential area, as the area experienced a 36% population growth         There are, however, some slight overestimations of the peak flow especially during the calibration period. Such

Impacts of CC and LUC scenarios on river flow
The response of the Wamkurumadzi River discharge to CC and LUC for the entire period and three decadal intervals (1980s, 1990s and 2000s) in each of the seven scenarios are shown in Figure 11 and Table 7. The two cases (CC and LUC) were considered both separately and integrated.
Two scenarios were consequently needed to evaluate their  respective impacts on river discharge: scenarios SC1 (as a baseline) and SC2 and SC4 (as a baseline) and SC5 (Table 7) were compared, respectively, for the LUC impacts only; scenarios SC1 (as a baseline) and SC3 and SC4 (as a baseline) and S6 (in Table 7) were compared for CC only.
On the other hand, scenarios SC1, SC4 and SC7 were then used to evaluate the integrated impacts of LUC and CC.
The results show that both CC and LUC had varying impacts on the river discharge which were also decadedependent. Under SC2, the results show that LUC alone resulted in a reduction of the mean river discharge by 11.67% in the 1980s. However, the student's t-test results showed that there were no significant differences in the mean discharges at α ¼ 0.05 level during the baseline scenario (SC1, mean discharge ¼ 3.17 m 3 /s) and S2 scenario simulations (mean discharge ¼ 2.8 m 3 /s). Consequently, LUC did not result in significant changes in mean river discharge between 1980 and 1990 in the catchment. Under scenario 5 (SC5), LUC alone resulted in increased mean river discharge by 16.56% between the 1990s and 2000s.
The t-test results showed that this LUC-induced change in river discharge was significant at α ¼ 0.05 level. The 1990s period coincides with a considerable reduction in the forest-covered area and a sprawl of the urban area ( Figure 3).
In many areas, studies in similar environments (e.g. Guzha On the other hand, CC alone (SC3) resulted in decreasing the river discharge by 49.8% between the 1980s and 1990s and increased the river discharge by 108.61% between the 1990s and 2000s (SC6). In both scenarios SC3 and SC6, the CC-induced changes in river discharge were statistically significant at α ¼ 0.05 level. Overall, the contributions of CC to river discharge can be attributed to changes in the rainfall regime where a relatively high rainfall period in the 1980s was followed by relatively lower rainfall with lower temperatures in the 1990s which subsequently picked up again in the 2000s. Figure 12 shows the inter-decadal variations of the mean annual rainfall and discharge from the 1980s to 2015s demonstrating this aspect.
In addition, the combined impacts of CC and LUC were represented by scenarios SC1, SC4 and SC7. The results show that annual mean river discharge decreased from   On the other hand, the mean annual temperature has been decreasing whereas the mean annual discharge has been increasing, both without statistical significance. In spite of the insignificant changes in the rainfall and temperature regimes, the river responded significantly to the climate variability, especially changes in the rainfall. These results highlight the sensitivity of the Wamkurumadzi River to climate forcing mechanisms and have significant consequences for water resources management for the area.
According to model evaluation criteria composed of statistical indices and graphics, the model's performance during calibration (1984-1999) and validation (2000-2015) periods was very good. The model outputs showed that the climate forcing had larger impacts on the historical river discharge as compared to LUC. In part, the reforestation measures introduced in the basin have helped attenuate the LUC impacts.