Soil Erosion Assessment in the Nepal Himalayas Using Models, Erosion Plots and Spatial Techniques

Abstract

<p>Soil erosion is a global environmental threat: it washes away fertile topsoil and reduces agricultural productivity. Being a mountainous country with accidented topography and intense rainfall in the monsoon, Nepal has a significant erosion issue. The quantification of soil loss is an important challenge in order to understand the changes in erosion severity across landscapes, land-uses and regions so that suitable soil and water conservation programs can be prioritised and implemented. This PhD study mainly focused on quantifying soil loss rates in the Middle Hills region of Nepal, as this region has higher rates of soil loss than other parts of the country. Specific objectives were to: (1) explore the causes and impacts of soil erosion in Nepal as well as management options to combat soil erosion; (2) calculate gross and net soil loss across significant parts of the country; (3) assess the impacts of tillage and mulch on soil erosion; (4) examine erosion dynamics in relation to long-term land-use change, and (5) assess gaps in erosion research in Nepal and assess the efficacy of the nation’s current policies to control land degradation, including soil erosion. This study of soil erosion used both qualitative and quantitative approaches, including literature reviews, field observations and erosion modelling. Data for analysis were obtained from Nepal’s Department of Hydrology, the Nepal Agricultural Research Council and the International Centre for Integrated Mountain Development, and were supplemented by field data.</p> <p>The thesis contains six research chapters and eight chapters in total. The first chapter provided a general introduction, overview and background to soil erosion on a global scale, as well as a particular focus on the Nepal Himalayas. The second chapter addressed land degradation by soil erosion in Nepal. It reviewed the causal factors, status, and management options in relation to soil erosion. Conventional agricultural practices characterised by excessive tillage operations, intense and erratic rainfall events in the monsoon season and undulant topography are the main drivers of soil erosion in Nepal. Soil erosion reduces agricultural productivity, pollutes land and water resources, and ultimately causes a loss in farm income. Mulching, cover cropping, strip planting, contour farming, conservation tillage and bioengineering are some of the practices used to combat soil erosion in the country. The national periodical plans and policies have generally been ineffective in achieving anticipated targets. Thus, Nepal needs to develop realistic evidence-based plans, focusing more on local participation and capacity enhancement.</p> <p>The third chapter quantified gross rates of soil loss in the Aringale Khola Watershed in Nepal, using the Revised Universal Soil Loss Equation (RUSLE) model. A mean annual soil loss rate of 11.17 t ha^–1 year^–1 was estimated for the study area. Topography had the greatest influence rate of soil loss since erosion increased with increase in percent slope. Forest was the dominant land-use covering 72% of the study area and was the greatest source of soil sediments (80%). </p> <p>The fourth chapter estimated sediment yield and maximum outflow in the Sarada River basin using the Intensity of Erosion and Outflow (IntErO) model. The study estimated an asymmetry coefficient of 0.63, indicating the likelihood of severe floods in the near future. Likewise, a maximum outflow of 1918 m³ s^–1 and a gross rate of soil loss of 10.74 Mg ha^–1 year^–1 were obtained from the analysis, which compare well with erosion plot measurements.</p> <p>The fifth chapter evaluated the impacts of no-till farming and mulch on soil and nutrient losses and yield in corn fields in the Middle Hills of Nepal. The results showed no-till farming significantly lowered soil losses. Likewise, the combination of no-till and mulch reduced losses of soil organic matter and total nitrogen. Soil loss was positively correlated with total seasonal rainfall, the monsoon being the most severe for soil erosion. The efficacy of mulch in reducing losses of soil organic matter and total nitrogen was higher in cultivated plots than no-till plots. Experiments of greater duration are required, ideally in areas with differing soil types, to understand the longterm effects of tillage and mulch on soil fertility and crop productivity across Nepal.</p> <p>The sixth chapter investigated the impacts of land use land cover changes (LUCC) on soil erosion dynamics in the period 1995–2015 in western Nepal, using RUSLE and a Geographic Information System. Erosion was greatest in agricultural lands and least in forest in each of the three target years. The Siwalik Hills and Middle Mountains were the most eroded zones among the different physiographic regions in the study area. Increase in agricultural lands at the expense of bare lands and forests have increased soil loss rates. The research confirmed the significant impact of LUCC on soil loss in the study area. </p> <p>The seventh chapter systematically reviewed soil erosion studies in Nepal published since 1 January 2000 to examine research gaps and evaluate the efficacy of current national statutes and policies. Most studies were concentrated in the central and western regions of Nepal, with the eastern and far western regions most neglected. The Agriculture Development Strategy and Land Degradation Neutrality program recently launched by the Government of Nepal (GoN) emphasise land and water conservation in different ways. The government’s periodical plans to date have suffered from weak institutional capacity to combat soil erosion across the country. We suggest that the GoN establish a national soil erosion database by collating all past and present soil-related studies. A separate policy specifically focused on combating natural disasters and land and water degradation, including soil erosion, is needed.</p> <p>This thesis aimed to provide a comprehensive overview of soil erosion modelling and employed different methodological approaches including field-based observations, literature reviews and analysis, and powerful yet simple modelling techniques such as RUSLE and IntErO. The results clearly suggest that soil erosion is a severe problem in the Nepal Himalayas and has been increasing over time. RUSLE was effective in estimating the soil erosion severity across the country, particularly for modelling gross rates of soil loss. However, IntErO was required to calculate sediment yield (net erosion) and delivery to rivers and streams. Erosion plots are important for examining erosion severity and comparing modelling results from the aforementioned models. GoN policymakers are urged to formulate a specific policy for soil erosion. This study has important implications for relevant government agencies such as the Nepal Ministry of Agricultural Development and its partner bureaus, especially in designing national policies and formulating suitable soil and water conservation plans to reduce soil erosion across the country.</p>