DETERMINATION OF WATER BALANCE COMPONENTS OF A MICRO WATERSHED FOR IMPROVED WATER MANAGEMENT PRACTICES

Abstract

The incidents of water shortage are prevalent even in areas with high annual rainfall and are a serious issue requiring immediate attention. The uneven temporal distribution of rainfall, poor water conservation, and management are the primary reasons for this scarcity. A water balance study on a watershed basis can provide objective solutions to water scarcity and other water management issues. Hence, such a study on a sub-watershed of river Bharathapuzha, having a catchment area in and around Valanchery municipal town in Malappuram district, has been carried out in this research work. Major objectives of the study included the determination of monthly water balances of the watershed, developing relationships between them, and suggesting scientific water management practices for the watershed. Four automatic tipping bucket rain gauges were installed in the study area to record the rainfall. The stream runoff was determined by the area-velocity method, using automatic water level sensors, data loggers, and cup-type current meters. The soil moisture was measured using both capacitance and resistance-based soil moisture sensors, and a water level sensor recorded the groundwater level. The evapotranspiration (ET) was estimated using the FAO Penman-Monteith (PM) method and the water balance method. The sub-components of water balance were also assessed using the Soil and Water Assessment Tool (SWAT). The SWAT-CUP (Calibration and Uncertainty Programme) was used for sensitivity analysis and calibration of the model. The monthly and weekly relationships were found between the rainfall and other water balance components using R software. The soil and water conservation measures were suggested for the study based on the morphometric and water balance information.

The average annual rainfall of the study area was found to be 2582 mm. Runoff measurement showed that 58 per cent of rainfall was transformed to runoff, while ET of the basin was 40 per cent. Soil moisture depletion rate in summer months varied between 2.45 to 1.39 mm day-1, while the mean monthly GW depletion rates were 3.74, 2.84, and 5.5 mm day-1 from December to February, respectively. The ET estimated by the PM and water balance methods showed a close comparison. The SWAT model showed that its predictive ability for runoff was good with the R2 value of 0.97, NSE 0.96, PBIAS 2.60, and RSR 0.20. The monthly relationships between rainfall and other elements of water balances were non-linear, with polynomial equations of degrees 3 to 5.

There was no flow even at the outlet of the mainstream of the watershed during the summer months from March to May. If the flow rate in the channel could be reduced during the later part of the monsoon and post-monsoon months (October to December), a lean flow of 1 m3s-1 in the stream can be ensured. Groundwater recharge in the upper reaches of the catchment can further improve the lean flow. Roof water harvesting, recharging, bunding, trenching, terracing, and other vegetative measures have been suggested for the upper catchment. Pervious check dams, cement concrete check dams and vented cross bars were suggested for the main drainage channels.