Study on the prediction of sediment yield from the hilly catchment Chittagong tracts

Abstract

An evaluation has been made between predicted sediment yield using available prediction fonnula and field measurement. Prediction formula used in this study is "Revised Universal Soil Loss Equation (RUSLE) which is a universally accepted formula over the world. For the purpose of evaluation, six experimental plots are established at Banderban of Chittagong Hill Tracts. This location has been selected in order to assess the soil erosivity of the hilly area for both conservation and non-conservation practice in ground. Among these plots three having conservation practice with hedge row and remaining three does not have any conservation practice. A hedge is a special type of plant having growth rate faster than normal plant, which is used to reduce the soil loss. In this study three-hedge row is used in each experimental plots. For the purpose of using prediction formula, various parameters such as the factor erosivity index (R), erodibility index (K), slope length factor (L), and slope steepness factor (S) are standardized utilizing the available measured data by fitting the results obtained from prediction formula. However the cover management factor C and the conservation practice factor P are calibrated by using the sediment yield obtained in the field measurement with that of the sediment yield obtained by prediction formula. In order to collect short duration rainfall data a modern aerodynamic rain gauge recorder having a tip sensitivity of 0.2 mm is used. Having been obtained all the parameters the prediction formula has been modified in this study for a particular type of soil and slope. This modified form of prediction formula can be used with reasonable accuracy for calculating sediment yield of a small plot for sheet and rill type erosion. Recently collected data for the year 1999 and 2000 has been utilized for calibration purpose. The sediment yield obtained by prediction formula and field measurements are shown in both tabular and graphical form. In the light of the comparison made between the sediment yield obtained by both the method, it is seen that the prediction formula can be applicable to the country like Bangaladesh with reasonable accuracy. It is also seen that the percentage of discrepancy between measured and predicted sediment yield varies from -28.62% to 26.486%. Again it is found that the use of conservation practice reduces the sediment yield by 33.73% for gentle slope, 48.05% for moderate slope and 51.02% for steep slope. From this study it is also seen tbat the value of erosivity index (R), maximum 30 minutes consecutive intensity (I30)

event wise erosivity index (Eho) and storm energy (Eg) can be determined without using the prediction formula and the places where auto raingauge recorder is not available. From the analysis it is also found that the maximum amount of sediment yield occurred in the steep slope without any conservation practice that is 61.424 mton/ha and minimum amount of sediment yield occurred in case of gentle slope, which is 14.25 mton/ha with conservation practice. However this prediction is limited to the size of the experimental plot and local rainfall pattern.