The understanding of soil erosion processes and the development of accurate erosion prediction models require understanding of detachment, deposition, and sediment transport in rills. The objectives of this study were to determine whether sediment transport capacity is a unique value for given soil, flow rate, and slope, and to determine if equilibrium sediment concentration in the rill obtained by detachment was different from that observed under depositional conditions. Experiments on a Carmi loam (fine, mixed, mesic Typic Hapludalf) simulated rill erosion under net detachment and net deposition conditions. Two discharge rates of 6 and 9 l min À 1 and two sediment input regimes of 0 and excess of transport capacity were tested on soil beds with lengths of 2, 4, 6, 7, and 8 m at 7% slope. Sediment load reached steady state conditions within the 8-m distance on the rill. At 9 l min À 1 discharge, 8 m length, and excess sediment added to the flow, sediment delivery was 71 g l À 1 versus 31 g l À 1 for the corresponding case with no sediment added. Overall, for the conditions tested, rill flow transported two times more sediment than it could detach. The flow did not reach its maximum potential transported load through detachment of soil due in part to changes in the sediment size distribution under deposition and possibly to the protective action of bedload particles moving along the rill bottom and/or changes in flow turbulence associated with sediment laden flow. D

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Numerous studies have shown the effectiveness of riparian buffers in reducing sediment, pathogen, and nutrient loads into surface and groundwater in agricultural catchments. Reported retention rates of sediment, N, and P were as high as 97%, 85%, and 84%, respectively. Often, however, riparian buffers fail to perform their protective functions due to low adaptability of their designs to local settings. This is caused by our inadequate understanding of the conditions under which riparian buffers perform the best at field scale. Therefore, a precision oriented approach based on thorough analysis of spatially variable characteristics of landscape has to be undertaken in riparian buffer construction. Such an approach has a potential to improve the protective qualities and the economic viability of the riparian buffers. This paper gives an overview of the current level of research on riparian buffers and discusses the importance of spatial variability of local conditions on their performance. It presents the approaches for precision buffer design and its practical implementation and highlights the directions for future development of precision conservation. Key words: riparian buffer, vegetative filter, water quality, precision conservation.

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[1] This study presents analysis of 34 years of precipitation, runoff, and sediment data collected from eight small (1.1-4.0 ha) semiarid rangeland watersheds in southern Arizona, USA. Average annual precipitation ranged between 354 and 458 mm with 53% of the total rainfall occurring from July through September. Runoff depth was 3.5%-13.9% of annual precipitation depth for individual watersheds and 9.2% on average. Runoff events with missing sediment data were estimated to account for 30% of the total sediment yield. Sediment yields were highly variable, ranging between 0.85 t ha −1 yr −1 and 6.69 t ha −1 yr −1 with an average of 2.4 t ha. Ten percent of rainfall events with the largest sediment yields produced over 50% of the total sediment yield during the 34 year period. Linear regression models were developed to relate precipitation and runoff characteristics to watershed sediment yield. Maximum 30 min precipitation intensity was the primary factor affecting runoff, and runoff was the best predictor for sediment yield, explaining up to 90% of its variability. Fire and drought may have significantly altered the hydrologic and sediment response on some of the watersheds, but lack of continuous monitoring of vegetation on the watershed areas complicated interpretation of both fire and grazing management effects.