Assessing water availability in a semi-arid watershed of southern India using a semi-distributed model
Résumé
Appropriate groundwater resource management becomes a priority for the States of the semi-arid southern India. Because of the highly increasing groundwater demand, the number of drought-prone regions where the groundwater resource is classified as over-exploited by the Government is critically increasing. Thus there is a need to develop quantitative methodologies adapted to the regional context that are capable to assess water resources at watershed scale and the impact of management measures. This study demonstrates the calibration and use of an integrated water resource assessment model (SWAT) in an 84 km(2) representative semi-arid crystalline watershed of southern India with no perennial surface water source. The model can reproduce (i) the recharge rate estimates derived independently by a groundwater balance computation, (ii) runoff and surface water storage occurring in tanks that spread along the drainage system, (iii) groundwater table fluctuations monitored at a monthly time step. Results show that even if the calibration period (2006-2010) was more humid than long-term average, the watershed is sensitive to the monsoon inter-annual variability with water-stress during the dry years and an associated loss in agricultural production. The impact of these dry years is spatially variable with higher vulnerability for sub-basins having proportionally larger irrigated paddy areas, lower groundwater resource, and/or lower recharge potential (i.e., due to land use and repartition of percolation tanks). The scope for additional recharge by means of managed aquifer recharge structures is limited and demand-side management measures are needed to mitigate pumping. A wishful management objective may be to see groundwater reserves as a supplementary resource in case of monsoon failure and not as the main water resource to be used indiscriminately. SWAT proved to be an adequate modeling framework for the simulation of water resource in semi-arid hard-rock context where groundwater vertical fluxes largely prevail over regional lateral flows at km-scale. It provides interesting capabilities for water availability mapping and the simulation of different types of scenarios (e.g., land use changes, climate change.