THE EFFECT OF ANTECEDENT MOISTURE CONDITIONS ON SEDIMENT AND PHOSPHORUS LOSS DURING OVERLAND FLOW: MAHANTANGO CREEK CATCHMENT, PENNSYLVANIA, USA

Authors: MCDOWELL, R - AGRESEARCH LIMITED; Sharpley, Andrew

Submitted to: Hydrological Processes
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/30/2001
Publication Date: 10/30/2002
Citation: MCDOWELL, R.W., SHARPLEY, A.N. THE EFFECT OF ANTECEDENT MOISTURE CONDITIONS ON SEDIMENT AND PHOSPHORUS LOSS DURING OVERLAND FLOW: MAHANTANGO CREEK CATCHMENT, PENNSYLVANIA, USA. HYDROLOGICAL PROCESSES. 2002. V. 16. P. 3037-3050.

Interpretive Summary: Phosphorus loss from soil to surface waters via overland flow is subject to chemical and physical controls that interact with one another to determine phosphorus availability and transport. Recent work has identified physical processes such as erosion and soil antecedent moisture content that are critical to the potential loss of phosphorus from a watershed. We hypothesized that phosphorus loss in overland flow is primarily a function of soil moisture conditions and its effect upon soil breakdown and erosion. We tested this hypothesis with simulated rainfall for two soil types and different soil moisture conditions in the laboratory and tested if similar processes occur in the field. Of the two soils studied (Berks and Watson), more erosion and phosphorus loss occurred from the soil with a lower organic matter content (Watson), particularly when it was dry, as often occurs in crop production. Thus, management should be directed at minimizing not only the build up of phosphorus in soil but the potential of extreme soil moisture variations particularly for the Watson soil. This could include the use of cover crops or no-till practices.

Technical Abstract: The loss of P in overland flow from most cultivated soils is controlled by erosion, and in turn, soil moisture. We evaluated the effect of soil moisture on erosion and P transport in overland flow by applying rainfall (7 cm/hr) to packed soil boxes (1 m long and 0.15 m wide) and field plots (1 and 10 m long by 1 m wide) of silt loams in a central Pennsylvania (USA) watershed. Flow from packed soil boxes took longer to initiate as antecedent soil moisture decreased from field capacity (2 min) to air dried (8 to 9 min). Even in the more complex field plots (i.e., soil heterogeneity and topography), the wetter site (1 by 10 m plot; 70% field capacity) produced flow more quickly (3 min) and in greater volume (439 L) than the drier site (1 by 10 m plot; 40% field capacity, 15 min, and 214 L, respectively). However, less suspended sediment was transported from wetter soil boxes (1.6 to 2.5 g/L) and field plots (0.9 g/L) than drier boxes (2.9 to 4.2 g/L) and plots (1.2 g/L). Differences are attributed to the site's potential for soil aggregate breakdown, slaking, and dispersion, which contribute to surface soil sealing and crusting, as dry soils are subject to rapid wetting (by rainfall). During flow, selective erosion and antecedent moisture conditions affected P transport. At field capacity, DRP and PP transport varied little during overland flow; however, P transport from previously dry soil decreased rapidly after the initiation of flow (6 to 1.5 mg TP/L), due to the greater slaking and dispersion of P-rich particles into flow at the beginning than end of the flow event. These results indicate that soil moisture fluctuations greatly affect erosion and P transport potential and that management to decrease the potential for loss should consider practices such as conservation tillage and cover crops, particularly on areas where high soil P and erosion coincide.