Rainfall erosivity and variability in the Northern Ethiopian Highlands
Introduction
Besides its importance for agriculture, precipitation is the driving force of most water erosion processes, through detachment of soil particles and creation of surface runoff (Moore, 1979). Rain is also a triggering factor for mass movements.
Due to the presence of numerous topographic obstacles for dominant winds, orographic rain is common in many parts of Ethiopia, especially in the Rift Valley. However, convective movements of air masses, caused by differential heating of the earth surface, and resulting rains of high intensity and often short in duration are most widespread in Ethiopia (Krauer, 1988).
There are various rain regimes in the different regions of Ethiopia. Attempts to regionalise rain patterns were made by Suzuki, 1967, Troll, 1970, Gamachu, 1977, Goebel and Odenyo, 1984. The variability of rain patterns also results in yearly rain depths being almost independent from elevation, at country scale (Krauer, 1988). Above 1500 m a.s.l., other factors, including slope aspect and characteristics of dominant air masses, mask the possible relationship between elevation and mean yearly rain. Eklundh and Pilesjö (1990) regionalised the explanatory factors of rain depth, using principal component analysis. Results show that in most regions elevation is not an explanatory factor. Unlike elsewhere (e.g. Marquinez et al., 2003), no in-depth studies of explanatory factors of rain distribution have ever been conducted at local scale in Ethiopia, though this is of utmost importance for predictions of crop productivity and rates of land degradation processes.
Rain erosivity is a function of the rain's physical characteristics. In tropical regions rains are intense. These characteristics, as well as rain depth, drop size distribution, terminal fall velocity, wind speed and rain inclination, determine rain erosivity (Obi and Salako, 1995). Raindrop sizes have been measured and analysed in many countries. However, to our knowledge, in Africa such studies only exist for Zimbabwe (Hudson, 1965, Kinnell, 1981) and Nigeria (Kowal and Kassam, 1976, Aina et al., 1977, Lal, 1998). Drop size distribution, for a given intensity, is unimodal and slightly skewed to the left (Brandt, 1990). Often, a relationship of the typeis expected between the median volume drop diameter (D50) and rain intensity (I), with a and b being constants for a given region (Hudson, 1971). This type of relationship is however questioned for high intensities, since raindrops have a maximum size (Hudson, 1971).
The calculation of volume specific kinetic energy of a raindropinvolves transformation of average drop diameter into mass (m), assuming that raindrops are spherical, and an assessment of the terminal fall velocity (v) of raindrops of different sizes, as experimentally obtained by Laws (1941). Calculations of kinetic energy generally show that there is an increase up to an intensity of about 75 mm h−1, above which Ekvol remains constant (Wischmeier and Smith, 1958, Hudson, 1971, Jayawardena and Rezaur, 2000, Salles et al., 2002). Measurements of kinetic energy and raindrop sizes are in most cases not readily available; hence the development of empirical relationships between rain intensity and kinetic energy, which is in general expressed on a volume-specific basis, in J m−2 mm−1. Time-specific kinetic energy (Ektime), which is related to Ekvol bygives generally better correlations with rain intensity (Salles et al., 2002).
A recent literature review (Van Dijk et al., 2002) learns that most studies relating rain erosivity to drop size were carried out in regions located at a maximum of a few hundred metre a.s.l., with some rare exceptions reaching 2250 m a.s.l. (Blanchard, 1953). Consequences of lesser fall height for rain drop size in mountain areas are hence not taken into account.
In Ethiopia, calculations of erosivity parameters rely entirely on I−Ekvol relations established in other parts of the world, and especially on the Wischmeier and Smith (1958) equation based on data from a few stations in North America (Van Dijk et al., 2002). Renard et al. (1997) no longer recommend that equation.
The objectives of this study are to investigate (a) the spatial variation of rain in the study area, and how this is influenced by elevation, slope aspect and gradient, as well as geographical position; (b) the temporal variations of rain; and (c) rain erosivity in terms of the different controlling parameters, such as rain intensity, rain drop size and kinetic energy.
Section snippets
The study area in the Northern Ethiopian Highlands
The climate of Ethiopia is complex: ‘Within short horizontal distances, climates from tropical to subhumid, and subtropical to arctic can occur’ (Krauer, 1988). For a given altitudinal level precipitation decreases and seasonality increases with latitude.
During the winter in the Northern hemisphere, the Intertropical Convergence Zone (ITCZ) is situated to the South of the equator in Eastern Africa. At this time of the year, the western Highlands of Ethiopia receive hot and very dry winds from
Daily rain
Over three rainy seasons, the tipping bucket rain gauge recorded 204 rain events (taking into account that the gauge did not function during 1 month) of 15 min of duration at least and were separated by 30 min at least. These 204 events were classified by the time (hour) of their start (Table 2). Forty seven percent of the events start in the afternoon and 30% in the evening (18–24 h), providing 84% of total rain. It hardly ever rains much before noon. This daily rain pattern is explained by the
Conclusions
Despite the short observation period (6 y, and some stations only 2 or 4 y), several conclusions can be drawn. In the 80-km2 study area, topographical aspects such as steep overall slope gradients (expressed by the presence of important cliffs), valley aspect, but not elevation, control the spatial distribution of annual rain depth. A non-linear multiple regression model (Eq. (6)), represents this rainfall distribution. Rain depth is highest in those places where air masses flowing through
Acknowledgements
This study is part of research programme G006598 funded by the Fund for Scientific Research—Flanders, Belgium. Financial support by the Flemish Interuniversity council (VLIR, Belgium) is acknowledged. Thanks go to Berhanu Gebremedhin Abay for continuous field assistance and monitoring of the rain gauge network. Numerous farmers, the local Agricultural Office, REST (Relief Society of Tigray) branch and the authorities of the concerned villages and district facilitated the research. Many thanks
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