Effects of land use change and climate change on irrigation water availability in the Mea Pheam catchment, Thailand
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The Mea Pheam River in the northern part of Thailand is the main supply of irrigation water for the hill tribes that live within its catchment. The catchment lies in the Huay Nam Dang National Park which is a matrix of highly diverse habitat types. However, the natural system within the catchment is under pressure and the availability of irrigation water for the inhabitants and their crops is decreasing, which could be an effect of land use change (LUC) and climate change (CC). The aim of this study is to provide a better understanding of the processes of LUC, CC, and their effect on the irrigation water availability in the Mea Pheam Catchment (MPC), with a focus on the effects of expected LUC and CC in the future. Following the aim, this master’s thesis provides a scenario-based quantitative model analysis of the irrigation water availability in the MPC. The main research question is: What is the current availability of irrigation water in the Mea Pheam catchment and will land use change and climate change affect this in the future? The possible changes were translated into four scenarios: (1) LUC in 2030 and 2060, (2) CC in 2030, (3) CC in 2060, and (4) a combination of LUC and CC in 2060. Scenario 1, 2 and 3 each have two sub-scenarios to represent LUC in 2030 and 2060, and the Intergovernmental Panel on Climate Change (IPCC) scenarios A2 and B1. The A2 CC scenario is seen as an extreme scenario and B1 as an intermediate. A model was developed to estimate the irrigation water availability of the four scenarios. The model results were assessed in respect to the annual present irrigation water availability. Calibrating the model was not possible due to the lack of long term discharge data. Validation of the model by comparing discharge measurements, which were conducted during field research, with simulated discharges during the same time period (2 months of the dry season) of the present showed that the model might underestimate the discharges. The sensitivity analysis showed that the model is most sensitive for the precipitation input, while the precipitation data that was used had some significant deficiencies. This could explain the underestimation of the irrigation water availability. Nevertheless the model was able to give estimations of the irrigation water availability of the 4 scenarios. The effects of LUC and CC on the future irrigation water availability came forward by assessing the scenarios hydrographs and flow duration curves (FDC) in respect to the present. In conclusion, the annual irrigation water availability of the present is roughly 2.2*107 m3. Land use changes are expected to lead to a decrease of irrigation water availability -only- in 2060 of 3.7%. Climate change is expected to lead to a decrease of availability in 2030 and 2060 of 1.2% and 8.3%, respectively. Both changes combined (scenario 4) decrease the availability in 2060 with 14.6% in respect to the present. The FDCs of all 4 scenarios showed that the discharges with a high exceedance probability (90%) decrease relatively the most during the dry season, with percentages varying between 30% and 70%. This further stresses the irrigation water availability during dry periods. Comparing the effects of LUC and CC, the latter negatively affect the irrigation water availability relatively the most. But only if the climate changes according to IPCC scenario A2, which is seen as an extreme CC scenario. Although the inhabitants of the catchment have applied adaptive forms of agriculture in the past. The simulated decreasing availability of irrigation water shows that the catchment´ ability to provide irrigation water will decline in the future, especially during the dry seasons, which forms a very serious threat for its inhabitants knowing that their livelihoods depend on the availability of irrigation water and agricultural adaptations has its limits.