Modelling the hydrological effects of land-use change in small catchment

Son, Ill (1990) Modelling the hydrological effects of land-use change in small catchment. University of Southampton, Doctoral Thesis.

Record type: Thesis (Doctoral)

Abstract

In addition to a review of the literature relating vegetation and hydrology it is possible to summarise results from earlier research, by using 43 studies on vegetation change and storm runoff reported from 65 individual catchments. These results were used in a regression analysis to construct a general model concerning the effects of land-use change upon storm runoff. Although this provided a general indication of the relationship there are several limitations in the data for so that the model derived from the regression analysis could not be employed to produce sound estimates of the hydrological consequences of land-use changes.

The conceptual lumped model was therefore adopted as the alternative to the regression model. One of the major advantages of the model is its ability to simulate extended flows. A further advantage is that this model requires only conventional, readily available hydrological data(rainfall, evaporation and runoff). These two advantages commend the use of this type of model for analysis of the hydrological effects of land-use change in tropical areas. HYRROM, one of the versions of the IH lumped model, is used to optimize the data from three catchments in Zimbabwe and three experimental catchments in Kenya.

Several optimization techniques were developed and defined in the course of optimization, but the results indicated that a model which incorporates more precise definitions of each of the hydrological components and of the relations between them is required. Several relations were modified in order to reflect the hydrological characteristics of the Kenyan experimental catchments. The final model developed and adopted was constructed as an 11-parameter, 3-storage, 1-input option model.

Using a number of initial conditions, the model was tried to optimized to the data from three Kenyan experimental catchments. The model efficiencies were near to or more than 95% and the errors of total flow were less than 5% for 10 separate periods divided according to their land-use state in 3 Kenyan experimental catchments. The bias of the optimized models were tested by several techniques. The extended flows are simulated in the prediction mode using the optimized model and the data set of the whole series of experimental periods. They are used to analyse the change of water budget and the changes of daily high-and low-flow caused by land-use changes. Relative water use was decreased by about 30%, and the annual peak flows were nearly doubled as a result of a 46% decrease in rainforest caused by tea plantations in Catchment K13. Relative water use was increased by about 20% and annual peak flows were decreased

by about 30% as a result of a conversion from clcarcut to pine plantation in catchment Kl 1. But, the flow regime of Catchment K17 was not significantly changed as a result of a conversion from sporadic to intensive grazing.

Although the distributed model is the ideal one for the analysis of the relation between land-use and hydrology, the low availability of data in tropical areas necessitates a simpler and easier to apply model for these areas. Therefore, the applications of an existing simple model to broadly defined regions and the accumulation of results from a range of tropical environments is required to approach the serious problem of land-use change, especially deforestation and its hydrological consequences in tropical areas.

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