Challenges and Future strategies for Couserration and sustainable management of land resources in Bangladesh

be Q c q C ∑ = =1 (2) The Universal Soil Loss Equation (USLE) is often used to predict erosion in grid cells. Unfortunately, it is not well suited to this task. A modification of the USLE called the USLE-M can do the task better. Given an adequate ability to predict runoff, the USLE-M predicts event erosion better than the USLE. It also provides a mechanism, which enables the impact of upslope runoff on erosion in grid cells to be modeled. These features are illustrated by data from runoff and soil loss plots and erosion predictions for grid cells in a subcatchment in the Rocky Creek catchment, Queensland, Australia. where T is the total number of time units (e.g. minutes) in an event, qt is the runoff amount and ct the sediment concentration measured in each time unit. In the USLE-M it is assumed that cbe is dependent on (a) the kinetic energy per unit quantity of rain and (b) a measure of the peak rainfall intensity since the peak rainfall intensity tends to produce the highest sediment concentration and the highest runoff rate during a rainfall event. The USLE erosivity index for a rainfall event is EI30, the product of the total amount of rainfall kinetic energy expended on the ground during the rainfall event and maximum rain intensity recorded using a 30-minute time base. The kinetic energy per unit quantity of rain is given by E divided by the amount of rain that falls during an event (Be). Thus, if I30 is assumed to provide a measure of the impact of event rainfall intensity on cbe, the erosivity index for an event (Re) is given by INTRODUCTION The spatial variation in erosion is of interest to land and water quality managers. Erosion occurring in a part of a catchment or watershed has both on site and off site impacts. Changing the land use in one particular area may have not only consequences in that area but in other areas downslope and on the materials carries by runoff to rivers, streams and impoundments (dams, lakes). The Universal Soils Loss Equation (USLE) (Wischmeier and Smith, 1965, 1978) is often used in estimating rainfall erosion within catchments. The Agricultural Non Source Pollution model (AGNPS) (Williams et al., 1975) is one example where this occurs. AGNPS uses the USLE to estimate the amount of sediment that is available to a sediment transport model, which is used to model the movement of sediment and associated pollutants from the sites of erosion across the land to gullies, streams and rivers and then to the outlet of the catchment during a rainfall event. Unfortunately, the USLE was not designed for this sort of use. A modification of the USLE, the USLE-M (Kinnell and Risse, 1998), is an event-based model that is better suited to this task. Re = Qe I30 E/Be (3) Because the runoff ratio (QR) is given by QR = Qe/Be (4) Eq. 3 can be written as Re = QR EI30 (5) This is known as the QREI30 index (Kinnell, 1997). The USLE-M is the version of the USLE that uses the QREI30 index as its index of event erosivity: RUMe = QR EI30 (6) Figures 1 and 2 illustrate the gain in using the USLE-M rather than the USLE in predicting event erosion. In Figure 1, event soil losses from a 22.13 m long plot with a slope gradient of 19 % during 10 years (1935-1945) at Arnot (Ithaca), NY, are plotted against the EI30 index (Figure 1A) and the QREI30 index (Figure 1B) using logarithmic scales. Values for Z(log), the logarithmic form of the Nash-Sutcliffe (1970) model efficiency index (similar to the correlation coefficient) are also shown. In this particular example, the USLE operates at an efficiency of about 54%, the USLE-M at an efficiency of about 77 %. The USLE-M The primary difference between the USLE and the USLE-M is that no explicit consideration of runoff occurs within the USLE erosivity index where as there is an explicit consideration of runoff in the USLE-M erosivity term. The theory behind the USLE-M erosivity index is based on the concept that event erosion (Ae) is given by the product of runoff amount (Qe) and the bulk sediment concentration for the event (cbe); Ae = Qe cbe (1) Figure 2 shows how the efficiency of the two models varies with the hydraulic characteristics of the soil as measured by the gross infiltration ratio for runoff producing events (GIRrope) at 14 locations in the USA and one in Australia. GIRrope is calculated by: where