SPATIAL AND TEMPORAL SCALING EFFECTS ON HYDROLOGY AND PHOSPHORUS LOSS IN RUNOFF FROM PASTURES

Abstract

Runoff and water quality data from twenty two research projects that investigated phosphorus loss from pastures fertilized with poultry litter were summarized in a common dataset. The studies consisted of spatial and temporal scale variations which ranged from 0.5 to 80000 m2, and 1 to 355 days after litter application, respectively. The objectives of this study were: 1) to investigate the overall interaction of the various explanatory variables and their effects on the response variables: runoff depth, dissolved reactive phosphorus (DRP) concentration, DRP loadings, total P (TP) concentration and TP loadings, and 2) to investigate the spatial and temporal scaling effects on hydrology, and DRP and TP losses in runoff from pastures. Independent (explanatory) variables considered were plot size, hydraulic conductivity, poultry litter application rate, days after litter application, slope, soil test phosphorus (STP), alum amendment, grazing rate, pasture height, rainfall intensity, rainfall duration, and runoff duration. The summarized common dataset was then grouped into three datasets: complete, litter applied, and controls only. Backwards stepwise regression technique using PROC REG in PC SAS Version 8.2 was performed to analyze the effects of the variables. The final models with significant variables (P = 0.1) were then used to assess the relative usefulness of the variables in explaining the variability of the responses. Variables that significantly influenced and explained much of the variability in runoff (hydrology) were pasture plot size, hydraulic conductivity, rainfall duration, runoff duration, slope, litter P, and STP. Grazing significantly influenced hydrology (runoff) from litter applied and control pastures. DRP loss from pastures was significantly influenced by poultry litter, rainfall duration, pasture height, plot size, rainfall intensity, and runoff duration. Grazing rate and hydraulic conductivity were significant for DRP loss from pastures that received litter. Variables that significantly influenced and explained much of the variability in TP concentration from pastures were litter P, rainfall duration, runoff duration, plot size, and rainfall intensity. Slope, grazing rate and alum amendments were variables that significantly influenced TP loss from control pastures. Variables that had significant effects on DRP loadings were plot size, hydraulic conductivity, litter P, slope, pasture height, rainfall intensity and runoff duration. Litter P, runoff depth, slope, hydraulic conductivity, plot size and rainfall intensity were the variables that significantly influenced and explained the variability in TP loadings from pastures. Spatial scale effects existed for hydrology, TP concentration, and TP loading. Spatial scale effects were also observed for DRP concentration, with the exception of the control pastures. Runoff depth, DRP and TP loading increased with increasing plot size, while both TP and DRP concentrations decreased with increasing plot size and time after litter application. Water quality modeling efforts should integrate effects of variables that affect P loss from pastures by accounting for scaling effects on hydrology and P losses observed in this study.