Spatial and Temporal Variation in Edge-of-Field Runoff During Frozen Ground Periods.

Agricultural lands are important nonpoint sources of phosphorus (P) and nitrogen (N), yet edge-of-field nutrient transport from snowmelt or rainfall runoff upon frozen ground have been understudied. This study analyzed 10 years (2003–2012) of agricultural runoff data from the Pioneer Farm in Platteville, WI, during which large spatial and temporal variations in snowmelt total loadings were observed. Samples from these periods show large contributions to total nutrient losses, as up to 94% of annual total N and P loading at the University of Platteville’s Pioneer Farm occurs during frozen ground periods. In seven out of the ten years, over 50% of the total runoff occurred during frozen ground periods. Total N transport during snowmelt events annually ranged from 20% to 96%, with the majority of the nitrogen being transported by snowmelt events. P transport characteristics depended heavily on the form of P being characterized. In most years rain runoff events controlled total P transport, however the interannual variation ranged from 9 to 94%, and over the study period 51% of the total P transport occurred during frozen ground periods. Dissolved reactive P transport via runoff occurred to a much greater degree in most years during frozen ground periods. The largest degree of spatial variation for snowmelt dissolved reactive P occurred in 2003 when annual loads ranged from 0.00363 pounds per acre to 2.48 pounds per acre. In the 2010 water year the average dissolved reactive P runoff concentration during frozen ground conditions was 6.6 mg/L (max = 19 mg/L) and average nitrite+nitrate concentration was 1.9 mg/L (max = 12 mg/L), which were over five times the averages measured during non-frozen rainfall runoff conditions. This research highlights the importance of quantifying snowmelt mediated nutrient transport as a means to better understand nutrient inputs into the environment.