Eight runoff plots, located within a small catchment within the Walnut Gulch ExperimentalWatershed, southern Arizona, were constructed to test the argument that sediment yield (kg m − − − − −2 ) decreases as plot length increases. The plots ranged in length from 2 m to 27·78 m. Runoff and sediment loss from these plots were obtained for ten natural storm events. The pattern of sediment yield from these plots conforms to the case in which sediment yield first increases as plot length increases, but then subsequently decreases. Data from the present experiment indicate that maximum sediment yield would occur from a plot 7 m long. Analysis of both runoff and sediment yield from the plots indicates that the relationship of sediment yield to plot length derives both from the limited travel distance of individual entrained particles and from a decline in runoff coefficient as plot length increases. Particlesize analysis of eroded sediment confirms the role of travel distance in controlling sediment yield. Whether in response to the finite travel distance of entrained particles or the relationship of runoff coefficient to plot length, the experiment clearly demonstrates that the erosion rates for hillslopes and catchments cannot be simply extrapolated from plot measurements, and that alternative methods for estimating large-area erosion rates are required. Copyright Figure 1. Design of small plots.rates and travel distances of entrained particles, Parsons et al. (2004) proposed a conceptual model for soil erosion that leads to scale dependency in erosion rates. For interrill flow, they argued that this interaction would cause sediment yield initially to increase as area (length) increased, but subsequently to decrease. This paper reports on a field experiment to test this argument.
MethodologyEight runoff plots were constructed within a small catchment located within the Walnut Gulch Experimental Watershed, southern Arizona. The catchment was vegetated with a desert shrub community dominated by Larrea tridentata and Acacia constricta. Four of the plots (named Hardy, Costello, Pete and Morecambe) were designed to be the same size (2 m long by 1 m wide). Slight variations in the construction of the plots resulted in actual plot sizes ranging from 2·08 m 2 to 2·11 m 2 . The side and upper plot boundaries were constructed using aluminium flashing inserted into a shallow trench dug into the soil and buttressed by concrete either side of the flashing. On the downslope side of each plot a triangular metal sheet directed runoff from the plot into a reservoir, of approximately 100 litres capacity, buried into the soil. Within this reservoir was a 13-litre bucket into which flow first fell and then overflowed into the reservoir. The metal sheet and reservoir were covered so that no rain falling onto them could enter the reservoir, nor could any sediment be splashed onto the sheet or into the reservoir. The design of these plots is shown in Figure 1. Water and sediment collected in the reservoirs were measured following each runof...
Dryland ecosystems are often characterized by patchy vegetation and exposed soil. This structure enhances transport of soil resources and seeds through the landscape (primarily by wind and water, but also by animals), thus emphasizing the importance of connectivity – given its relation to the flow of these materials – as a component of dryland ecosystem function. We argue that, as with the fertile‐islands conceptual model before it, the concept of connectivity explains many phenomena observed in drylands. Further, it serves as an organizing principle to understand dryland structure and function at scales from individual plants to entire landscapes. The concept of connectivity also helps to organize thinking about interactions among processes occurring at different scales, such as when processes at one scale are overridden by processes at another. In these cases, we suggest that state change occurs when fine‐scale processes fail to adjust to new external conditions through resource use or redistribution at the finer scale. The connectivity framework has practical implications for land management, especially with respect to decision making concerning the scale and location of agricultural production or habitat restoration in the world's drylands.
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