TN-17 : Drought analysis using soil moisture simulation approach

Abstract

A number of drought indices have been developed in past. Most of these indices appear to be adhoc and consider either rainfall alone or take some account of soil water storage and loss of water from this storage by evapotranspiration. An operational definition of drought could best be one that compares daily rainfall values to evapotranspiration rates to determine rate of soil moisture depletion to give upto date status of available soil moisture after correcting for surface runoff, and expresses these relationships in terms of drought effects on crop growth at various stages of crop development. An attempt has been made in this paper to discuss a simple approach for development of a soil water budgeting model to simulate daily soil moisture in dry lands using historical rainfall and climatic data and moisture characteristics of the soil. The severity of drought for a given crop can be studied by defining different levels of drought definition (i.e. different levels of soil moisture content corresponding to different soil water deficits). The incidence of drought can be characterised by determining the number of days during the growing season of a crop when simulated soil moisture is below a value which is known to impede crop growth appreciably. This operational definition can be used to analyse drought frequency, severity and duration for a particular crop in a given watershed.