RN-4 : Rainfall recharge

Abstract

The natural hydrological phenomenon of rainfall recharge is very complex to study, analyse, and evaluate due to the unpredictable unbounded random variable nature of its input parameters in relation to the soil-vegetation-atmosphere system. Any work on the estimation of the recharge of aquifers by rainfall needs improved clear understanding of the dynamic coupling across the interface among the physical processes of the soil, vegetation, and atmosphere systems. Certain empirical as well as numerical studies were made to formulate inter- relationships among the principal variables that govern the phenomenon. Due to the weak physical basis of the variables, generalisation and parametric incorporation towards the solution of physical problems relating to the recharge or moisture transfer are difficult to be validated.

However, certain physically based representations/modelling of the various parameters related to this natural phenomenon have been reviewed in this study. The different approaches of study involved include establishment of empirical formulae under specific field conditions, evaluation by experimental studies, and analysis by combined deterministic and probabilistic approaches.

A detailed study has been made on the rainfall recharge process under the three main areas, empirical formulae, experimental studies, and combined deterministic and analytical studies. Different methods of study on rainfall recharge are outlined with the understanding of their basic concept and with coverage of all the different techniques available. Finally, five methodologies are elaborated and recommended to be adopted for solving physical problems under Indian conditions. The dynamic water balance study recommended covers poisson and gamma distributions of precipitation, soil moisture movement, evapotransiration, distribution of storm surface runoff, distribution of water yield, and a physically based water balance method of estimation of the rainfall recharge. The second method involves evaluation of natural infiltration/soil moisture variation by experimental studies including laboratory experiments. The third method indicates the rainfall/water table level relationship study by statistical time series analysis using recursive approach in the estimation of rainfall recharge. The fourth method illustrates a semigraphical method of estimation of recharge from rainfall-runoff relationship. The fifth method out-lines the derivation of series of two-dimensional equations for infiltration from which the recharge or deep percolation loss of moisture can be assessed.