WATERSHED HYDROLOGICAL MODELLING CONSIDERING LANDUSE AND CLIMATE CHANGES

Abstract

Limited and deteriorated land resources coupled with the change and variability of climate, associated impact and vulnerabilities are the growing environmental issues of the world in the 21st century. It is accountable for numerous effects on overall hydrological cycle, which significantly influences the climatic parameters viz., rainfall and temperature, hence it hinders the overall environment in general and agricultural productivity in particular. Therefore, clearly knowing the climatic conditions and the overall hydrological behaviour of the watershed is quite important to plan, manage and use the available resources in a sustainable way without compromising the resultant ecosystems. The present study is associated with investigating historical climatic data such as rainfall and temperature have been used for trend analysis and its associated effect to understand the long-term climatic situations of the Beressa watershed of Ethiopia. Land Use/Land Cover (LU/LC) change analysis and future prediction have been done in order to analyse the effects on runoff and sediment as well as on agricultural productivity. Soil and Water Assessment Tool (SWAT model) has been used to calibrate and validate runoff and sediment response to LU/LC change dynamics for Beressa watershed. Considering various scenarios viz., climatic trend and variability, LU/LC change dynamics, runoff and sediment process, sustainable integrated water resources planning and management and water harvesting has been suggested for the area. The results of classified images shown that farmland and settlement areas have been expanded at the rate of 71.60 ha/year and 16.80 ha/year respectively. The share of forest cover decreased at the rate of 5 ha/year in the year between 1984 and 1999. While water bodies has decreased at the rate of 0.03 ha/year. However, in the second study period the year between 1999 to 2015 the share of forest cover has been regenerated by the rate of 15.60 ha/year. Likewise, water bodies has regenerated at the rate of 7.10 ha/year. The share of grazing land has been reduced to 10% in 1999. Similarly, barren land has been reduced to 10.10% in the year 1999, the share further discriminate to 6.10% of grazing land and 5.20% of barren land in 2015 compared to 12.40% of grazing and 13.10% of barren land in 1984. The probability of farmland and grazing land may decline in the future (2015-2030). Due to increase in settlement areas (35.14%) and barren land (36.11%), which facilitate the decline in farmland (6.73%) and grazing land (11.53%), even though forestland and water body slightly increase, it is insignificant. Therefore, the future dynamics of LU/LC classes may contribute to destruction of natural resources and hence severely affect the availability of food production. ii Analysis of trend using MK test and Sen’s slope as well as spatial distribution using IDW have done at local level for both annual and seasonal rainfall and mean, minimum and maximum temperature for 35 years (1980-2014). The rainfall trend of Beressa watershed shows both increasing and decreasing trends. Significantly decreasing trend of mean annual rainfall was observed in HG station (-8.62 mm/year and -27.88%), whereas, significantly increasing trend of mean annual rainfall was observed in the DB station with magnitude of 0.28 mm/year and 1.07% in DB station. During rainy season (kiremit season) significantly decreasing trend was recorded in the SD station with magnitude of (-0.90 mm/year and -16.20%) while significantly increasing trend was observed in DB station (1.62 mm/year). During small rainy season (belg season), significantly decreasing trend observed in the GIN station with the magnitude of -0.12 mm/year whereas significantly increasing trend was observed in the DB station (0.40 mm/year). A decreasing trend of bega season rainfall were observed in all stations varied from -0.06 mm/year in GIN station to -0.19 mm/year and -56.40% in DB station. Rainfall during kiremit and belg seasons shows slightly moderate distribution as compared to annual and belg rainfall in which high concentration of rainfall distribution emerged. From this observation, rainfall distribution is classified as irregular to erratic distribution hence significant effects on overall crop production. The trend of mean, minimum and maximum temperature have been increased in all stations with magnitude of 0.95oC/35 year, 0.7oC/35 year and 1.1oC/35 year respectively. Physical based hydrological model known as Soil and Water Assessment Tool (SWAT) has been used in order to understand the hydrological behaviour of Beressa watershed. Based on the results obtained, the model has been successfully calibrated (1980-1999) and validated (2000-2014) using the observed weather, runoff and sediment input data. In order to calibrate the model, thirteen (six for surface runoff and seven for sediment yield generation) most sensitive parameters were adjusted within the ranges until acceptable agreement between observed and simulated values were obtained. The objective function for the model evaluation statistics observed that 0.72, 0.62, 0.52 and 3.90% for R2, NSE, RSR and PBIAS respectively during calibration period for runoff process whereas corresponding values during validation periods were 0.68, 0.64, 0.56 and 7.60% respectively. The model evaluation statistics during the calibration period showed 0.68, 0.71, 0.58 and 4.20% respectively for sediment yield whereas the corresponding values during validation period were 0.65, 0.67, 0.62 and 8.30 respectively. The value of calibration and validation results proved SWAT model is applicable to small watershed like Beressa watershed. Therefore, based on the calibration and validation parameters the model of runoff and sediment yield simulation provides assurance for further iii application of the model in order to assess the influence of LU/LC change dynamics on surface runoff process and sediment yield generation. The alteration of LU/LC classes during the year between 1984 and 1999 from forestland, barren land and grazing land into farmland and settlement areas have significantly contributed for the increases of surface runoff and sediment yield for 1984 land cover by 4% and 6.03% respectively. The change of LU/LC types between 1984 and 1999 from forestland, barren and grazing land into farmland and settlement areas significantly contributed for increases of surface runoff and sediment yield by 4% and 6.03% respectively for 1984 and during simulation results using 1999 land cover runoff and sediment further increased by about 6.50% and 8.20% respectively. During the study periods, increase of farmland and settlement areas by about 18.20% and 59.70% respectively, consequently runoff process and sediment yield by about 158.10 mm/year and 4332 t/year respectively. Generally, LU/LC change dynamics significantly affect runoff and sediment yield of Beressa watershed. Consequently, an option for hydro-climatological scenarios consideration was developed in order to allocate and sustainable uses of limited natural resources viz., land resources, water resources, and other resources. CROPWAT 8.0 model and Rainfall Contribution Index (RCI) have been used to describe the adverse effects of climatic variability. The analysis showed that crops growing during small and dry rainy season (February to June) have high water requirement, even during rainy season due to high rainfall concentration, the available rainfall is not sufficient for the whole growing periods hence less productivity. It is noted that, in the face of high competition among natural resource users where there is significant climatic variability, rainfall shortage, water scarcity, and other resources shortage in the watershed, supplemental irrigation, water harvesting and integrated water resources management, and equitable, sustainable and efficient resources utilization have always persisted a social and economic goals. Despite some common limitations, this research is important to support the planning and development of water harvesting, adaptation of climatic variabilities and integrated water resources management for the Beressa watershed. Keywords: Climate change; climatic variability; Mann-Kendall; IDW; LU/LC; Supervised classification; Markov Chain Mode; SWAT model; CROPWAT 8.0; RCI; IWRM.