Process-based Investigation of Sediment Transport and Deposition in Irrigation Dominated Watersheds
Gregory B. Pasternack. Peng Gao, Wesley W. Wallender, and Khaled Bali
The purpose of this study was to investigate the physical processes that control sediment movement in agricultural drain channels and to establish the empirical model describing the variation of the sediment load.
The study area was limited to the Holtville main drain system in the Salton Sea basin in the Imperial Valley of arid Southern California, whose east part was a linear channel network and west part was a dendritic network. An extensive turbidity and stage monitoring program based on grab sampling using 500-mL plastic bottles and in situ monitoring using both turbidity sensors and pressure transducers was designed to represent spatial and temporal heterogeneities of sediment loads. The obtained data included (1) weekly stage and turbidity at 15 sites, (2) 5-minute stage and turbidity at 6 sites, (3) random turbidity and sediment concentration in irrigated fields and drain channels, (4) intermittent stage and discharge at 12 sites, and (5) daily stage and turbidity at one site. The turbidity and stage values were subsequently converted to corresponding sediment concentrations and discharges using various established turbidity-concentration relationships and stage-discharge relationships. At weekly time scale, mass balance approach was taken to distinguish erosion and deposition processes in drain channels at various spatial scales, which were then verified by cross section measurements and analysis of spatial distribution of sediment loads.
Based on detailed analysis, a model describing the process of sediment transport at various spatial scales was developed. It revealed that drain channels at larger scales tended to be dominated by deposition, while those at smaller scales were inclined to be controlled by erosion at the weekly time scale. Further analysis on sediment dynamics at the 5-minute scale exposed that at small-scale drain channels, deposition occurred in a very short period of one week (about one day) with high rate, though the remaining period was dominated by less intensive erosion. Therefore, regarding both spatial and temporal variations, sediment transport in the Holtville sub-drain system was generally controlled by deposition. This provided a scientific reason for the necessity of channel dredging, which has been regularly conducted by IID.
Investigating the impact of various agricultural practices (i.e. land use, irrigation methods, land cover, and soil texture) and channel morphology change on the process of sediment transport indicated that land use and irrigation methods are two most important factors that affect sediment transport. Any implementation of best management practices needs to concentrate on the variation of these two factors. The significant disturbance of multiple factors other than water discharge on sediment transport prevented any reliable sediment rating curve from being developed for drain channels at small and meso spatial scales. As an alternative, the empirical models for sediment transport at three different time intervals were developed at the large spatial scale. However, the nature of sediment transport -the high variation of sediment load within a narrow range of discharge variation determined that the models failed to provide accurate estimation of sediment load for a given discharge. It is therefore not reliable to use an empirical model in terms of three-month data to evaluate sediment load in the rest of a year. Even so, the results indicated that process-based study is still critical to understanding sediment dynamics at various spatial and temporal scales. The study also suggest that estimating sediment load using data on a monthly base is not accurate at all as the monthly sampling missed many important sediment variations occurred at finer time scales.
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