Biophysical considerations in integrated catchment management : a modelling system for Northern Thailand

Abstract

A trend in environmental modelling and policy making has been to move towards the use of integrated assessment methodologies as a means of balancing what are most usually multi-issue problems with multiple stakeholders. These methodologies are particularly relevant for the management of water resources given the interdisciplinary nature of water problems. This thesis presents a Biophysical Toolbox that integrates models for assessment of the outcomes of biophysical scenarios of water and land use options. Outcomes are measured in terms of environmental indicators that are outputs of the models. The Biophysical Toolbox is comprised of three modules - the CATCHCROP crop model, a hydrologic module based on the IHACRES rainfall-runoff model, and a modified Universal Soil Loss Equation (USLE) approach. The models were designed to have the following features: basic data requirements with which to drive the models; relatively low parameterisation requirements thus limiting problems with error accumulation; physically plausible, and easily transferred to other catchments. The aim with integrated models of this type is to be able to discriminate between, and be confident about, the relative changes in indicator outputs. The CATCHCROP crop model was applied to subcatchments of Mae Chaem in northern Thailand to identify: the impact of land management options (irrigation, fertilisation and bunding status) on model outputs; and the sensitivities of deep drainage, surface runoff, and crop yield estimates to the values of CATCHCROP model parameters. Model behaviour was shown to be plausible when the land management of the crop was varied. Results of a sensitivity analysis demonstrated the considerable non-linearity in the response of CATCHCROP outputs to parameter values, and identified which parameters most influenced the behaviour of the model. Two parameters were found to be substantially insensitive. Estimates of potential erosion were sensitive to the choice of equations describing the topographic and rainfall erosivity factors of the USLE. Despite this, the choice of equations should not affect the utility of the Biophysical Toolbox where direction and relative values of indicators between scenanos are important. Similar values of the erosion susceptibility term in the USLE were obtained for each land unit whether the procedure was applied on each land unit or on a more detailed grid-basis. Testing of the hydrologic model involved analysing: the performance of the regionalisation methodology in predicting discharge; changes in hydrologic response under forest cover changes; and sensitivities in the model outputs to changes in values of CATCHCROP model parameters. The regionalisation procedure generally overestimated annual discharge although is capable of capturing relative changes. As forest cover decreases, the proportion of streamflow in the quick flow component increased, annual and wet season discharge increased, and dry season discharge decreased. The relative quick and slow flow volume components were strongly sensitive to changes in forest cover. Six main parameters within the CATCHCROP model were identified as greatly impacting the calculation of quick and slow flow components. The Biophysical Toolbox was then used to examine deforestation, climate, and land management scenarios for the Mae Uam subcatchment of the Mae Chaem catchment, highlighting tradeoffs among indicators and raising questions about perceived impacts. The main results of the scenarios can be summarised as: deforestation scenarios do not greatly impact streamflow although increases in potential erosion are extreme; upland rice and vegetable crops are particularly susceptible to 'unacceptable' rates of erosion on steeply sloping lands; wet season crop yields are more dependant on plot fertility than on irrigation or bunding status. Sensitivity analysis of the whole toolbox was also performed. Despite potentially large impacts of CATCHCROP parameter changes on the re-calculation of the hydrology parameters, this did not translate into large differences in total seasonal discharge. Comprehensive testing of the Biophysical Toolbox illustrated the potential, and the plausible behaviour, of the toolbox for exploring aspects of land and water resource management in catchments in northern Thailand. Most useful to improve confidence in model performance would be further testing of the hydrologic module in instrumented catchments that have undergone significant forest cover changes over the period of hydrologic record. Show more