Katsuyuki Shimizu

Dynamics of Soil Erosion as Influenced by Watershed Management Practices: A Case Study of the Agula Watershed in the Semi-Arid Highlands of Northern Ethiopia.

Since the past two decades, watershed management practices such as construction of stone bunds and establishment of exclosures have been widely implemented in the semi-arid highlands of northern Ethiopia to curb land degradation by soil erosion. This study assessed changes in soil erosion for the years 1990, 2000 and 2012 as a result of such watershed management practices in Agula watershed using the Revised Universal Soil Loss Equation. The Revised Universal Soil Loss Equation factors were computed in a geographic information system for 30 × 30 m raster layers using spatial data obtained from different sources. The results revealed significant reduction in soil loss rates by about 55 % from about 28 to 12 t ha−1 per year in 1990–2000 and an overall 64 % reduction from 28 to 10 t ha−1 per year in 1990–2012. This change in soil loss is attributed to improvement in surface cover and stone bund practices, which resulted in the decrease in mean C and P-factors, respectively, by about 19 % and 34 % in 1990–2000 and an overall decrease in C-factor by 29 % in 1990–2012. Considerable reductions in soil loss were observed from bare land (89 %), followed by cultivated land (56 %) and shrub land (49 %). Furthermore, the reduction in soil loss was more pronounced in steeper slopes where very steep slope and steep slope classes experienced over 70 % reduction. Validation of soil erosion estimations using field observed points showed an overall accuracy of 69 %, which is fairly satisfactory. This study demonstrated the potential of watershed management efforts to bring remarkable restoration of degraded semi-arid lands that could serve as a basis for sustainable planning of future developments of areas experiencing severe land degradation due to water erosion.

Response of streamflow to climate variability and changes in human activities in the semiarid highlands of northern Ethiopia

Climate variability and human activities are two major drivers influencing changes in streamflow response of a watershed, and thus assessing their relative effect is essential for developing sustainable water resources planning and management strategies at watershed-scale. In this study, a runoff model driven by rainfall and potential evapotranspiration was established to estimate the effect of climate variability on the changes in annual streamflow of Agula watershed in northern Ethiopia. Significant decreasing trends were observed for annual and wet season streamflow between 1992 and 2012, while dry season streamflow showed an increasing trend. Analyses of seasonal and annual rainfall records showed no significant trends. The change-point test revealed that an abrupt change in annual streamflow occurred in 2000. In the period 2000–2012, the mean annual and wet season streamflow decreased by 36 and 49%, respectively compared with 1992–1999, while dry season streamflow increased by 57%. Climate variability was estimated to account for 22% of the total reduction in mean annual streamflow, whereas human activities (e.g., proper watershed management practices and associated changes in land use/land cover among other factors) were responsible for 78%; indicating that human activities were the major drivers of changes in the streamflow response. The results of this study point to the potential that reduced wet season flow and improved dry season water availability can be achieved by proper planning and implementation of appropriate watershed management practices.

Spatial distribution and temporal trends of rainfall and erosivity in the Eastern Africa region

Soil erosion by water is one of the main environmental concerns in the drought-prone Eastern Africa region. Understanding factors such as rainfall and erosivity is therefore of utmost importance for soil erosion risk assessment and soil and water conservation planning. In this study, we evaluated the spatial distribution and temporal trends of rainfall and erosivity for the Eastern Africa region during the period 1981–2016. The Precipitation Concentration Index (PCI), Seasonality Index (SI) and Modified Fournier Index (MFI) have been analysed using 5×5 km resolution multi-source rainfall product (Climate Hazards Group InfraRed Precipitation with Stations, CHIRPS). The mean annual rainfall of the region was 810 mm ranging from less than 300 mm in the lowland areas to over 1200 mm in the highlands being influenced by orography of the Eastern Africa region. The PCI and SI revealed a spatial pattern of rainfall seasonality dependent on latitude, with a more pronounced seasonality as we go far from the equator. The MFI showed high spatial variability with about 55% of the region subject to high to very high rainfall erosivity. The mean annual R-factor in the study region was calculated at 3246 ±1895 MJ mm ha-1 h-1 yr-1, implying a potentially high water erosion risk in the region. Moreover, increasing and decreasing trends of annual rainfall and erosivity were observed, but with substantial spatial variability for both. This study offers useful information for better soil erosion prediction as well as can support policy development to achieve sustainable regional environmental planning and management of soil and water resources.

Quantitative analysis of reusing agricultural water to compensate for water supply deficiencies in the Nile Delta irrigation network

The main problem facing farmers in the Nile Delta is water shortages at the ends of irrigation networks and canals. These problems have worsened as water demands have increased. Egypt’s Ministry of Water Resources and Irrigation is currently trying to avoid water deficits by returning agricultural drainage water to the irrigation canals. In the Kafr El-Sheikh Governorate, some canals have an oversupply in some months and deficits in others. Ministry officials started a project by constructing culverts connected canal ends with the main drain (Bhr Nashrat) to provide supplemental agricultural drainage backflows (SADB) channeled through these culverts. However, this return is not controlled, and flows are based only on differences in the hydraulic head. In this study, we evaluated the effectiveness of SADB to counteract water shortages when the water supply from head regulators (WSHR) is insufficient. Our analysis considered the adequacy of the water supply and indicators of dependability and equity of supply. We tested two water supply conditions: (1) WSHR only and (2) WSHR plus SADB. During the summer (May–September) of 2008, SADB significantly improved the system’s ability to meet the irrigation water requirements (IWR) in some months. Adequacy and dependability, therefore, improved from “fair” to “good.” During the following winter (October–April), SADB improved adequacy and equity only in March and April, since water availability was generally sufficient under WSHR.

Irrigated Agriculture and Salinization

The Loess Plateau has complex topography with deep, thick loess, and is poor in water resources. The soil is loose and rich in carbonates, but because these dissolve readily when moistened, the soil is susceptible to subsidence and erosion damage when wet. These natural characteristics have hindered irrigation development on the plateau. Irrigation methods on the plateau can be classified into three categories: (1) gravity irrigation on flat tablelands in the southern region, (2) gravity irrigation on steep topography, and (3) pump irrigation on relatively flat, high plains. Among these three, (1) and a part of (2) have potential salinization hazards, with no such potential for category (3) and most of category (2). We analyzed the relationship between topographic characteristics and potential salinity hazards of irrigated farmlands in the Luohui Irrigation District, a typical large-scale irrigation system located on the southeast Loess Plateau. Based on onsite observation, we summarize current and potential problems of irrigated farmland within a semiarid area. Salinization of irrigated farmland is greatly influenced by water management. The construction, operation, and management of a well-conceived and elaborate water management system is a requirement for sustainable agriculture.

Secondary Salinization and Its Countermeasures

We selected for study the eastern block of the Luohui Irrigation District, which is called the Luodong block. The research area is in the southeast part of the Loess Plateau, which is in a semiarid climate zone. Furrow and border irrigation in the block have been practiced over about 32,000 ha of irrigated area, where crops such as cotton, wheat and fruit are predominant. Based on onsite experiments in the research area, we classify salinization processes, reveal causes of farmland salinization, and propose countermeasures to prevent these phenomena. Lowering groundwater level by improving drainage systems, controlling dumped saline soil, and managing the use of saline irrigation water are recommended.