Spatial patterns of soil erosion and deposition in two small, semiarid watersheds

Abstract

[1] This work investigates spatial patterns of hillslope erosion and sediment yields in a semiarid ecosystem considering influences of vegetation, slope, rocks, and landscape morphology. The 137 Cs inventories were measured on one shrub and one grassed watershed in southeastern Arizona. Calculated mean erosion rates in eroding areas were 5.6 and 3.2 t ha À1 yr À1 , and net erosion rates for the entire watershed, including depositional areas, were 4.3 and nearly 0 t ha À1 yr À1 for the shrub and grass watersheds, respectively, over the past four decades. Differences in hillslope erosion rates between the two watersheds were apparently due to vegetation: while on the shrub site, runoff pathways were unobstructed, on the grass site, runoff was obstructed by vegetation patches and litter. Hillslope erosion rates within the watersheds were not correlated to slope gradient or curvature but were correlated to rocks in the upper soil profile. These results are interpretable in terms of slope-velocity equilibrium wherein overland flow velocities became independent of slope gradient because of differential rock cover, which evolved as a result of preferential erosion of fine material on the steeper slopes prior to 137 Cs deposition. Watershed morphology and channel incision controlled sediment yield. Most of the eroded soil was deposited in swales of the grassed watershed. Most of the soil eroded in the shrub watershed was exported from the watershed outlet by way of a well-incised channel system. The study shows that measurement of sediment yield from a watershed can be a poor indicator of erosion taking place within the watershed.Citation: Nearing, M. A., A. Kimoto, M. H. Nichols, and J. C. Ritchie (2005), Spatial patterns of soil erosion and deposition in two small, semiarid watersheds,

Winter cover crops as green manure in a temperate region: the effect on nitrogen budget and yield of silage maize

“…Kirkby et al (2004) describes a non-linear spatial and temporal response to climate change, with relatively large increases in erosion during wet years compared to dry years, and sporadic increases locally. Nearing et al (2005) showed that erosion increases with increases in precipitation amount and intensity, while erosion decreases with increases in ground and canopy cover. These results are consistent with the expectation that erosion should increase as the main driving force -rainfall -increases.…”

Effects of antecedent soil moisture on runoff modeling in small semiarid watersheds of southeastern Arizona

“…Watersheds 63.102, 63.103 and 63.104 are drained by well-developed, incised channel networks that efficiently deliver eroded particles to the watershed outlets (Nearing et al, 2005). Watershed 63.106 is smaller than the others and does not have a highly incised channel, but as with the other three also does not have a toe-slope area of noticeable deposition and sediment storage (Polyakov et al, 2009).…”

Slope–velocity equilibrium and evolution of surface roughness on a stony hillslope

“…Given that the slope characteristics had been largely determined prior to the time period spanned by the field experiment with the 137 Cs (Nearing et al, 2005), it is reasonable that more rock at a sampling point correlated with erosion less than would otherwise have occurred between 1963 and 2004, which was the approximate time span representative of the erosion estimates. The energy (and hydraulic shear) of flow available for erosion and transport of sediment was reduced as a function of increased hydraulic roughness of soil surface cover (rocks) because of the increased energy lost on the rougher surface (Nearing et al, 2001).…”

“…Correspondent interrelationships between flow velocities, flow rates, and slope gradients have not been investigated for interrill or sheet flow conditions. Nearing et al (2005) hypothesized that stony hillslopes in the semi-arid environment evolve to a state of "slope-velocity equilibrium". We define slope-velocity equilibrium as a state that evolves naturally over time due to the interactions between overland flow, erosion, and bed surface morphology, wherein steeper areas develop a relative increase in physical and hydraulic roughness such that flow velocity is a unique function of overland flow rate independent of slope gradient.…”