Landform and Agricultural Management Characteristics Influencing C, N, P Export With Extreme Rainfall Driven Flows in the Assiniboine River Watershed

Abstract:

The Assiniboine River Watershed (ARW) drains 182,000 km² of Saskatchewan, Manitoba, and North Dakota. Peak flow rates observed in 2011 and in 2014 were the highest recorded since hydrometric measurement was initiated in 1913. Under high flow conditions the Province of Manitoba operates a water control structure at Portage la Prairie (the Assiniboine River Diversion; ARD) diverting water outside the ARW to Lake Manitoba (4,624 km²) to prevent flooding in densely populated areas near and in the City of Winnipeg. Recent research by Page (2011, 2015) indicates that in years of operation the ARD is the largest source of P and second largest source of N to Lake Manitoba. Along with alteration of the biogeochemistry of the Assiniboine River itself in flood years, the large influx of water from the ARD has led to less saline and more nutrient rich conditions in Lake Manitoba over the last 17 years (Page, 2011). For the May-June period in the ARW the average rainfall, number of consecutive days with rain >1mm, and proportion of rain occurring on days with rain rate in the 95^th percentile has increased significantly since 1960 and this pattern may be linked to anthropogenic climate forcing (Szeto et al., 2015). Land use throughout the ARW is predominantly agricultural and extensive changes in tillage practices, surface drainage, cropping systems, and fertilization have occurred over the last 50 years. As a result, understanding how agricultural land management influences biogeochemical responses to extreme spring precipitation events is of particular importance. In the research presented here we describe patterns of flow, C, N, and P export in 2014 for a network of 8 fields (2-6 ha.) and 9 tributaries (60-1000 km²) of the ARW as related to agricultural management and surface drainage patterns. Annual export from tributaries was controlled by surface water storage capacity in wetlands and depressions, as measured by effective contributing area (Fig 1) with increases observed for those watersheds with more extensive artificial drainage. Across the tributaries studied the extreme rainfall event accounted for 64% on average of overall water yield in 2014, but event specific water yield related more with antecedent wetness than to drainage. Responses at the field scale correlate closely to soil test N and P and to antecedent soil moisture.