Spatial modelling of mountainous basins; An integrated analysis of the hydrological cycle, climate change and agriculture

Abstract

Water is the most essential substance on earth and a changing climate has an important impact on the temporal and spatial distribution of water availability. Mountain ranges provide an important “water tower' function and over 20% of the global population depends on fresh water resources provided by the Himalayan range in critical periods of the year. The hydrological cycle is more intense in mountains. Mountains are also more vulnerable to climate change due the dependence of the surface hydrology on snow and ice melt, which directly responds to temperature increases. Although great advances have been made over the last decades in measuring and modelling the hydrological cycle at increasing temporal and spatial resolutions, scientific work in this field in mountain areas has however lacked behind. This study has taken a systems approach to the interaction between the hydrological cycle, climate change and agriculture in mountain catchments by contributing to four crucial topics. For the first topic knowledge on the spatial and temporal precipitation patterns across different land uses on the Tibetan plateau was enhanced. A time series of normalized difference vegetation index was manipulated using a Fast Fourier transformation. The manipulated signal proved to yield interesting information about the interaction between vegetation and precipitation and the absolute amounts of precipitation. The second topic focused on the assessment of the effects of climate change for the Brahmaputra basin. The historical trends in precipitation and temperature from 1900 onwards were analyzed. Regression analysis of historical precipitation showed an interesting significant relationship between monsoon precipitation and the temperature difference between the Tibetan plateau and the low-lying floodplains. Outputs of general circulation models were statistically and spatially downscaled and ensemble averages revealed accelerated increases in precipitation and temperature that seemed to be positively related to altitude. Multiple regression analysis revealed that the downstream summer discharge is subject to a steep increase, with will most likely result in an increase in flooding in the low lying plains of Bangladesh. For the third topic an innovative methodology was developed to calibrate the process based semi-distributed hydrological model SWAT. Instead of the traditional way of using stream flow gauge data, remotely sensed actual evapotranspiration was used for the calibration. Different sets of soil, land use, and meteorological parameters were optimised and the calibrated SWAT model was then used to evaluate water use and water productivity in the Upper-Bhima catchment in southern India. The final component adds a link between biophysical and economic modelling. The “payments for ecosystem service” concept to conserve water was implemented in an agricultural catchment on the Tibetan plateau, which has an important water supplying role for downstream areas. It was shown that by providing farmers with financial incentives they may shift from irrigated to rain-fed agriculture as long as the compensation is high enough. This integrated approach has shown fascinating results, has clearly added value and opened many new scientific avenues for the future to be explored