Modelling the Surface Hydrology of Khet Systems in Nepal during Monsoon Storms.

Abstract

Population pressure and consequent land use change is perceived to be leading to an increase in khet (rice paddy) cultivation in Nepal. Theoretically, in monsoon storms khet systems should provide temporary stores for hillslope runoff, beneficially reducing the quickflow component of rivers and their susceptibility to flooding. This research examines this hypothesis through the development of a computer model to replicate and thus better understand the surface hydrology of khet systems. The KhetFlow model was conceived in a deterministic manner, based on a continuity equation describing change of water storage in khet terraces. Development of the model is described through the structure advocated by Beven (2012), whereby perceptions are hardened through perceptual and conceptual models to a working procedural model, to be then calibrated and validated. Perceptions of the controlling processes were gained through general observation, a prototype model, a pilot study and extended fieldwork in Nepal which together with sensitivity analysis indicated that, as would be expected, the system was most responsive to rainfall (and rainfall enhanced irrigation inflow). Also of importance was the interaction between terraces, the division of the hillside into khet subsystems and the interaction between these subsystems. From a modelling point of view the most important control was the need to maintain a water balance throughout the system. The model was calibrated against field data collected in Nepal and found to be of acceptable accuracy. Using the values derived during calibration, the model was validated against different field data and then deployed in a predictive capacity to examine the hydrological behaviour of khet systems in a variety of plausible situations. These applications suggested that khet systems operate in an efficient manner during storms, even when storm frequencies are high; but have limited capacity to buffer storm water, even when initially dry. However, contrary to the original working hypothesis, very high levels of quickflow generation were predicted, particularly during heavier storms.