The effect of using different time steps in a soil water balance model to estimate groundwater recharge in the dry zone of Sri Lanka

Importance of groundwater as a natural, renewable resource is well documented. The rate of replenishment of water table (or rate of groundwater recharge) is a key issue central to sustainable development of this valuable resource. Although many workers have used a soil water balance to estimate groundwater recharge, not many workers have investigated the effect of using different time steps (i.e. a day, 7 days etc.) in the water balance on the estimates of recharge. This study looks at the effect of using weekly, 10 daily and monthly time steps in a soil water balance to estimate recharge in the dry zone of Sri Lanka. The findings suggest that the larger the time step, the lower the recharge estimate as found in a different study in Grimsby in UK (Howard and Lloyd, 1979). Combining the results of documented studies and the current study, it is shown that wherever possible, a daily time step needs to be used in a soil water balance and if daily evapotranspiration data is not available, the available data may be evenly distributed (e.g. say weekly data divided by 7) and used as daily data in order to arrive at reasonable estimates of recharge together with actual daily rainfall data, which are likely to be available in most parts of the world