Three models were applied to quantify the rate of runoff in the Nam Chun watershed and assess the effects of different land use/ cover types on its generation. The index approach was simple, required very little data, and its application in a spatially distributed modelling environment allowed for estimates of runoff rates at different locations in the catchment. Though simple this approach, it was deemed unsuitable for this particular study. The reason for this was that, the approach made no considerations on the physical laws that govern the processes involved in the generation of surface runoff. It also did not consider accumulation of lost rainfall in the catchment which, under natural conditions would influence the amount of rain water lost in each time step. Since data for model validation was not available in this study, making conclusions based on results obtained from purely black box modelling approaches that are not validated with ground truths could be misleading. It could lead to decisions that do not coincide with the reality on the ground. With adequate data however, a good approximation of the impacts of different land use/ cover types on the rate of runoff could be obtained easily using this approach which could be used as a start of runoff investigations in an area.
The second approach was the SCS curve number approach. Like in the previous case this method is not data demanding and also took into consideration the effects of different land use/ cover types and soil characteristics of the catchment in predicting runoff. The results obtained were similar to those of the semi physical approach. It showed that areas of agriculture practice have higher excess precipitation values than non-agricultural areas. However the empirical basis of the equations used in this method and the lack of long term discharge data for validation made this approach also unsuitable for this study.
The final approach was a semi physical based approach. Most of the equations used in the model were based on the physical laws that govern the components of surface runoff. It was therefore deemed the most suitable approach for this study because it was assumed that its physical basis would allow it to make predictions of surface runoff as it would occur in nature. It was also considered the most appropriate approach for this study because it explicitly considers the effects of different land use/cover types on the generation of parameter values used in the model.
The results of the study revealed that areas of agricultural practice could be concerns of soil degradation problems that are resulting in increased rates of surface runoff in the watershed. It was observed that in areas of agriculture practice, soil properties related to the infiltration of water into the soil were being negatively influenced due to the land use/ cover practices being implemented in these areas. The consequence of this was seen in the high rates of surface runoff being generated in areas of agriculture practice. The scenario studies further confirmed that changes in land use/cover to agricultural practices could lead to significantly altered outcomes in the discharge pattern of the river Huai Nam Chun which could lead to severe flooding problems in low lying areas.
The study showed that PCRaster was a valuable tool to quantify the rate of surface runoff and assess the impacts of different land use/ cover types on runoff generation. Its flexibility in allowing the application of different runoff modelling approaches (one of which was based on the integration of equations from different sources) in the context of the Nam Chun Watershed made it an effective means to assess the impacts of different land use/ cover types on environmental problems such as surface runoff. It enabled the integration and analysis of large data sets that vary both spatially and temporally which emphasised the practicality of employing GIS tools for assessing dynamic environmental problems. Therefore, it can be concluded that PCRaster was a suitable tool for making the assessments this study set out to achieve.
Recommendations
The results of all the modelling approaches indicated that there is a trend in the surface runoff pattern of the watershed on different land use/ cover types. Areas used for agriculture exhibited higher surface runoff rates than areas of non-agriculture. The soil analysis results also further substantiated this indicating that soil properties associated with the generation of runoff generally were being negatively influenced by agricultural practices. Since the study revealed that there is a cause of concern in areas of agricultural practice, further investigations should be conducted in the catchment. Perhaps through the collection of adequate model validation data and making field measurements of the parameters required by the model more accurate predictions can be made. Because of its physical nature, the model can then be implemented in other catchments in the area. With this information, suitable management strategies can be formulated and implemented in an effort at reducing the rate of surface runoff ultimately reducing soil erosion by water and the flooding hazard in the low lying areas.
Development of Methodologies for
Land Degradation Assessment Applied to
Land Use Planning in Thailand
