Hydrological and hydraulic modelling of runoff-producing areas in small rural catchments

Abstract

Runoff processes and contributing areas have been investigated using hydrological and hydraulic models formulated on different scales. Procedures developed for modelling runoff-producing areas in small catchments are presented in three major sections: Hydraulic models for runoff processes on a hillslope scale: analytical solutions to equations of groundwater flow on an inclined impervious base subject to time-varying recharge have been obtained using triple mathematical transforms. Analytical solutions have also been obtained for the recharge rate and loss rate. Using the computed recharge processes based on readings of watertable, runoff patterns, namely infiltration excess or saturation excess, can be distinguished at particular locations by comparing the peak times of stream discharge and recharge rates with respect to timing of rainfall. Analytical expressions for saturation zones on hillslopes have also been obtained. It has been shown that the variations of the recharge rate and saturated zones have the same geometric shape, indicating the role of recharge in reshaping groundwater. It has been shown that seven factors affecting the saturation zones or seepage face are: (1) the recharge rate; (2) the porosity of soil; (3) the transmissivity of the medium; (4) the depth of the soil; (5) the gradient of the land; (6) the gradient of the impervious layer; and (7) the initial depth of the watertable. Hydrograph analysis on a catchment scale: new parameters and standards based on hydraulic and hydrological principles have been defined for hydrographic geometry on a catchment scale. By definition, a hydrograph consists of overland flow and baseflow, and a hydrograph of baseflow is further divided into two parts, the rising and recession limbs, each represented by a hydrological model. The time of concentration of overland flow is used as a criterion for timing the end of surface flow. It is proposed and verified that the baseflow peaks and the overland flow ceases after the overland flow travel time of the 'last drop of rain'. Then, the lag time of the baseflow with respect to rainfall or total stream runoff can be derived. Individual and complex hydrographs can be separated into baseflow and overland flow using the models. Unit hydrograph theory has been extended to baseflow hydrographs which yields reasonable results. Models for runoff producing areas on a catchment scale: an improved isochrone model for saturation excess from a catchment is used to derive procedures for assessing the areas producing saturation runoff. It is simple enough for practical use. Overland flow separated from the hydrographs has been input into this model to yield the areas generating overland flow. Geomorphological approaches have been developed for dynamic stream networks which allow the saturation areas of a catchment to be evaluated and mapped. Show more