ASSESSMENT OF FACTORS CONTROLLING EPHEMERAL GULLY EROSION IN SOUTHERNPORTUGAL AND CENTRAL BELGIUM USING AERIAL PHOTOGRAPHS

Ephemeral gully erosion in agricultural land is an important source of sediment that is frequently being overlooked and not accounted for in soil erosion studies. However, there is field evidence that ephemeral gully erosion is responsible for significant soil losses. On the othe r hand, little is known about the factors controlling ephemeral gully erosion. In addition, the relative contribution of ephemeral gully ero sion to total sediment production in agricultural catchments has not b een assessed. The aim of this study is to assess the relative importan ce of ephemeral gully erosion in two different environments. In this s tudy we use aerial photographs to estimate the relative importance of ephemeral gully erosion in topographically defined now paths in the se diment budget of some study catchments. These study catchments are loc ated in Southern Portugal with a dominance of lithosols and Central Be lgium where loam and sandy loam soils prevail. The field data indicate that erosion in topographically defined drainage lines is the dominan t sediment source and causes the major erosion threat in the loam belt of Belgium and for the stony soils in the Alentejo region of south Po rtugal; the ratio between the soil loss due to ephemeral gully erosion and rill-interill erosion ranges between 0.4 and 2.3 for the Belgian study sites and between 4.5 and 5.2 for the Portugese catchments. Ther efore, the importance of erosion in drainage lines has to be taken int o account when planning erosion control measures at the catchment scal e. The observed pattern of ephemeral gully erosion can not be explaine d by using the available rill erosion equations. However, in the loamy region of Belgium the pattern of ephemeral gully erosion can be predi cted by using the product of upslope contributing area (A) and the slo pe gradient (S) which is a measure for the erosivity of the concentrat ed flow. In Portugal however, the pattern of ephemeral gullies can be predicted by a combination of A.S (measure for the erosivity of the ov erland flow) and a measure of soil saturation (In[A/S]). The soil prof ile is not very deep and is overlaying an impermeable rock layer. Thes e results indicate that erosion in topographically defined concentrati on lines in Belgium is mainly the result of Hortonian overland flow wh ile in Portugal soil saturation and related seepage forces seems to be the dominant incision mechanism.