Evaluation Of Irrigation Induced Hydrological Changes In The Malaprabha River Basin, Karnataka, India

Abstract

Water and food are the two most essential needs for the survival of humankind. However, satisfying their increased demands for a growing human population remains a major challenge for several countries. The increasing demand for food can be fulfilled to a large extent by enhancement in agricultural productivity through the introduction of irrigation. Consequently, there has been a worldwide increase in irrigated agriculture during the past several decades. However, the introduction of large-scale irrigation in a region can bring about a wide variety of changes in the environmental, economic, and social domains. Since irrigation water withdrawals account for more than 70% of the total available water resources of the world, there is a potential for alteration of hydrological processes and regional climate patterns. Environmental impacts of irrigation in general and hydrological impacts in particular, which are usually detrimental in nature, have been a cause for concern given the rapid increase in irrigated agriculture across the world. Therefore, studies aimed at assessing irrigation impacts on regional hydrology are very essential to understand changes in the hydrological cycle and the magnitudes of water balance components. Improved understanding of such impacts will pave the way for developing plans for sustainable development and management of water resources. The primary objective of the present research work was to analyze the impacts of large- scale irrigation on river-basin scale hydrological processes. Accordingly, the Malaprabha River basin located in a semi-arid Karnataka State, India in which an irrigation project was established in 1972, was selected for the study. The sequential methodology adopted to evaluate the hydrological effects of irrigation consisted of 1) Characterizing the river basin using historical observations of hydro-meteorological variables 2) Analysing the likely effects of irrigation on long-term trend and variability of hydro-meteorological variables 3) Analysing the historical growth of irrigated agriculture in the Malaprabha river basin using CROPWAT 4) Identify and evaluate the contributions of major drivers causing the stream-flow change in the river using the SWAT model and 5) Evaluating the ii hydrological impacts of irrigation using plausible cropping pattern scenarios in the river basin. With the obtained historical hydro-meteorological data (rainfall, rainy days, average temperature, stream flows, and groundwater levels), a preliminary analysis was carried out using box-whisker plots and Spatio-temporal maps over the Malaprabha river basin. The analysis revealed that the large part of the basin experiences annual average rainfall between 544 mm to 700 mm, which is a typical range for a semi-arid climate. Streamflow regime at downstream gauging stations was significantly affected by the Malaprabha irrigation project in the study area causing increased low flows (LFI upto 111%) during summer months and decreasing amount of peak flows (HFI upto 37.4%) during monsoon. Also, higher GWL fluctuations (10 to 20 mbgl) were observed in the downstream command area during all seasons. This defending the fact that excessive groundwater utilization for growing water-intensive crops in the immediate vicinity to the reservoir. Trend analysis for the historical hydro-meteorological variables was carried out from 1960 to 2015 using nonparametric Singular Spectrum Analysis (SSA) and conventional Sen’s slope Estimator (SE) methods. The results demonstrated the ability of SSA to capture the trajectory of nonlinear trends over the entire time series of hydro- meteorological variables. The traditional SE and MK methods, on the other hand, provide information on linear monotonic trends. The temporal variability of the data was analyzed using the Coefficient of Variation (CV) statistic. Variability study revealed that the presence of the reservoir has resulted in the occurrence of rainfall events with higher intensities in its vicinity. Also, wells located in irrigable command areas are subjected to greater variability. The trend analysis indicates non-significant decreasing rainfall and rainy days till the year 2000, but an increasing trend thereafter. A significant increasing trend in mean temperature was observed for all the stations and all the seasons of the basin with an average magnitude of 0.2⁰ C per decade. The annual stream-flow trends for downstream gauging stations were subjected to variability as these are regulated flows and showed decreasing trends corresponding with the progression of irrigation in the iii command area. Groundwater levels of most of the wells in the upstream region showed increasing annual trends. The two wells located in close vicinity of the Malparabha dam towards the downstream side showed significantly decreasing trends. On the other hand, the wells in the downstream command area indicated a combination of significantly increasing and decreasing trends. The analysis of the historical growth of irrigation in the Malaprabha command area revealed that the commissioning of the irrigation project has a significant role in the development of irrigated agriculture in the region. The contribution of canal supplies to irrigated agriculture was maximum until 1985-86 (61%) and decreased thereafter and the contribution of canal supplies to irrigated agriculture was maximum until 1985-86 (61%) and decreased thereafter. Also, the regions close to the reservoir appear to be fully benefitted by canal water supplies whereas regions located away from the reservoir seem to be benefitting from groundwater supplies. A shift from low water consuming crops to water-intensive crops is observed and the area under cash crops has increased significantly. Cropping-pattern violations, flood-irrigation, illegal water withdrawals, and poor maintenance of canal and associated structures are likely causing the current status. Overall, it appears from the performance analysis that the Malaprabha irrigation project has not been able to enforce the planned objectives and goals. The SWAT hydrological model was applied to study the combined and isolated effects of Malaprabha reservoir, LULC change, and climate change for the decades 1980s, 1990s, and 2000s. The combined effect of changes in all three drivers caused an increase in annual stream-flow in the basin by 53% between the 1980s and 1990s and a decrease in stream-flow by 38% between 1990s and 2000s. The study reveals that in a tropical river basin the presence of an irrigation reservoir can significantly alter temporal variability of stream-flow which is further exacerbated by changes in LULC and climate. On the other hand, the analysis of irrigation effect on stream flows revealed that when irrigation is withdrawn, water availability in the basin was found to be improved significantly. Also, increased low-flows during the non-monsoon period and decreased flows during the iv monsoon period have been noticed for irrigation conditions concerning no-irrigation conditions. The quantity of actual evapotranspiration (AET) in the study for existing irrigation conditions was increased by 4 to 26% concerning the no-irrigation scenario and 15% concerning the proposed irrigation scenario over the irrigated sub-basins. The present study demonstrated a sequential methodology adopted to evaluate the hydrological effects of irrigation over the Malaprabha river basin through statistical analysis as well as using a hydrological model. The information provided by this study will be useful in solving water scarcity issues in the river basin through the development of effective management strategies to improve the efficiency of the Malaprabha project and promote the sustainable development of natural resources in the study area.