Effect of Microtopography On Spatial and Temporal Rill Initiation Using Close Range Photogrammetry at Angereb Watershed, Ethiopia.

In Ethiopia rill erosion is a common symptom of erosion on the hill slopes of agricultural lands. Assessment of areas exposed to critical rill erosion is of utmost importance for soil and water conservation planning. Often, identification of these critical erosion sources has been explained in terms of flow hydraulic indices developed at laboratory condition. The difficulty in measuring hydraulic erosion processes during runoff events at field condition necessitates the use of spatial topographic and micro-topographic factors. Yet, the spatial rill formation and development are poorly understood at field condition where these factors are more complex, and occur at different intensities under Ethiopian climatic and tillage management conditions. This investigation was carried out to identify and determine the critical rill initiation source areas on different soil textures under conventional tillage induced soil surface roughness at field condition. 
The experiment was conducted from July to August 2007 on 3.7 m wide and 14.4 m long freshly tilled, bare plot at Angereb watershed, north of Lake Tana in Ethiopia.  The total rainfall in July to August was about 660 mm. The investigation was made before and after successive natural rainfall events.  Digital Elevation Models (DEM) of 2 cm grid resolution were compiled with the method of close range photogrammetry (CRP) using a calibrated digital single lens reflex camera with a wide angle lens. To investigate the effect of topographic and micro-topographic changes on rill initiation, DEMs were generated before and after the rainfall events.
Rill initiation was determined for each sampled rills using mean slope gradient (Scr, m m^-1), unit contributing area (Ac, m² m^-1) at head cut of rills and the random roughness before the rain (RR, m). Rill initiation parameters were predicted from non-linear regression, S_cr = a*A_c^b*RR^c. Critical rill areas were identified where S_cr*A_c^-b*RR^-c exceeds the constant, ‘a’, threshold area. Significant improvements were obtained by considering the random roughness of the soil surface. During the initial 42 mm of rainfall, critical rill erosion occurred where the threshold area exceeds 0.40, 0.48, 0.51, 0.34, and 0.21 for sandy loam, clay loam, loam, sandy clay loam, and clay soil textures respectively.