RESULTS AND INTERPRETATION OF SOIL LOSS MEASUREMENTS FROM STEEP SLOPES IN THE PHILIPPINES

Abstract Measurements of runoff-event soil loss and one-minute rates of rainfall and runoff are reported for runoff plots installed on the tropical Philippine island of Leyte. Plots were either under traditional crops cultivated using farmer practices, or kept bare. Plots were of length 12 m and at slopes of 50% to 70%. Soil loss for the cultivated crop was 35 t ha -1 y -1 , and 63 t ha -1 y -1 for the bare soil plots. An erodibility parameter β calculated for bare-plot data exceeded the value 1 for lower stream power events, indicating enhancement of flow-driven erosion by other processes, such as rainfall impact. This conclusion held whether an original erosion model was employed, or a subsequent model development designed to acknowledge the special effects of very high sediment concentrations and shallow flows common at the site

Visual comparison of spatial patterns of annual suspended sediment loads estimated by two water quality modelling approaches

The Queensland Department of Environment and Resource Management is using the SedNet and E2 water quality modelling approaches to support government policy and natural resource managers in improving water quality. SedNet is designed to determine the long-term average annual sediment load, and does not deal with temporal variability. It includes hillslope erosion, gully erosion, and riverbank erosion, which enables land managers to undertake on ground works in areas of the landscape that generate disproportionate quantities of sediment. E2 is a daily time step model capable of modelling temporal variability in water quality as a result of management and/or climate changes. However, hillslope erosion, gully erosion, and riverbank erosion are not currently explicitly represented in E2 in which sediment generation is based on the concept of Event Mean Concentration (EMC) and Dry Weather Concentration (DWC) with user assigned values depending on factors such as land use, soil type, and topography. As a modelling framework, E2 is capable of housing alternative models for the same process. Both SedNet and E2 modelling approaches are based on node-link configuration of the stream network generated from pitfilled digital elevation models. This configuration allows the user to determine outputs from either model at any point of interest within the catchment

Plot-scale rainfall-runoff characteristics and modeling at six sites in Australia and Southeast Asia

During major runoff events when most soil loss occurs, runoff is likely to dominate the rainfall-driven erosion processes. Thus accurate estimation of the runoff rate is critical to soil loss predictions. At plot scale, the Green-Ampt infiltration model is commonly assumed to be able to describe the temporal variation of the infiltration rate over a storm event. Field measurements of both rainfall intensity and runoff rate at 1-min intervals at six sites in the tropical and subtropical regions of Australia and Southeast Asia, however, strongly suggest that the apparent infiltration rate is closely related to the rainfall intensity and it is essentially independent of the cumulative infiltration amount, features not accord with the Green-Ampt infiltration equation. Furthermore, the storage effect and runoff rate attenuation are not negligible at the plot scale. With an initial infiltration amount to determine when runoff begins, an exponential distribution to describe the spatial variation in the maximum infiltration rate and a linear storage formulation to model the lag between runoff and rainfall, we were able to develop a satisfactory three-parameter model for the runoff rate at 1-min intervals within a storm event.During major runoff events when most soil loss occurs, runoff is likely to dominate the rainfall-driven erosion processes. Thus accurate estimation of the runoff rate is critical to soil loss predictions. At plot scale, the Green-Ampt infiltration model is commonly assumed to be able to describe the temporal variation of the infiltration rate over a storm event. Field measurements of both rainfall intensity and runoff rate at 1-min intervals at six sites in the tropical and subtropical regions of Australia and Southeast Asia, however, strongly suggest that the apparent infiltration rate is closely related to the rainfall intensity and it is essentially independent of the cumulative infiltration amount, features not accord with the Green-Ampt infiltration equation. Furthermore, the storage effect and runoff rate attenuation are not negligible at the plot scale. With an initial infiltration amount to determine when runoff begins, an exponential distribution to describe the spatial variation in the maximum infiltration rate and a linear storage formulation to model the lag between runoff and rainfall, we were able to develop a satisfactory three-parameter model for the runoff rate at 1-min intervals within a storm event

Use of GUEST technology to parameterize a physically-based model for assessing soil erodibility and evaluating conservation practices in tropical steeplands

A general motivation that led to the development of GUEST was to seek a more physically based soil erodibility measure than that provided by the K factor in the USLE. This motivation was strengthened by the finding that use of the USLE methodology in the extensive wheatgrowing belt in Australia, where annual soil loss is very variable, required several decades of experimentation in order to determine the K factor with useful accuracy. This made the methodology inappropriate in such contexts (Edwards, 1987). Furthermore, the technique used to determine soil loss in establishing the USLE, thought to be suspect, was later shown to lead to serious underestimation of soil loss, especially in well structured soils (Ciesiolka et al., 2006)

Toward a framework for runoff and soil loss prediction using GUEST technology

In recent years, a number of physically based models have been developed for soil loss predictions. GUEST is one such model based on fundamental physical principles and the current understanding of water erosion processes. GUEST is mainly used to determine a soil erodibility parameter. To apply the model in a predictive mode, the model is simplified in a physically meaningful manner for flow-driven erosion processes, and 2 essential hydrologic variables are identified, namely total runoff amount and an effective runoff rate. These variables are required to determine soil loss for individual runoff events. A simple water balance model was developed and used to predict runoff amount from rainfall amount. The efficiency of this runoff amount model in prediction was over 90% using field data. A 1-parameter regression model (r ~ 0.9) for the effective runoff rate was also established which uses peak rainfall intensity in addition to rainfall and runoff amounts. The prediction of peak rainfall intensity for a given rainfall amount and storm type was also sought. The field data were from Goomboorian, near Gympie, in south-east Queensland and these data were used to test and validate both models. Results overall are satisfactory and the approach adopted is promising. A framework for soil loss prediction is established within which individual parts can be further refined and improved