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TECHNICAL REPORTS

Surface Water Quality

Are Current Phosphorus Risk Indicators Useful to Predict the Quality of Surface Waters in Southern Manitoba, Canada?

^a Dep. of Soil Science, Univ. of Manitoba, Winnipeg, MB, Canada R3T 2N2
^b Institut National de la Recherche Scientifique, INRS-ETE, 490 de la Couronne, Quebec, Canada G1K 9A9

^* Corresponding author (Don_Flaten{at}UManitoba.ca).

Received for publication April 6, 2008. Many phosphorus (P) risk indicators have been developed to assess the risk of P loss from agricultural land to surface water. Most of these indicators are designed for land and climates where rainfall-induced erosion of particulate P from sloping landscapes is the main process of P transport. No indicators have been validated in the Canadian Prairies, where P losses are driven by snowmelt-driven runoff over nearly level landscapes and frozen soils. The objective of this project was to evaluate the relationship between water quality data for P from 14 watersheds and three existing P risk indicators for their potential use in the southern Manitoba prairie region of Canada. None of the indicators, including Birr and Mulla's P Index, a preliminary P risk indicator for Manitoba, and a preliminary version of Canada's National Indicator of Risk of Water Contamination by Phosphorus, was significantly correlated with mean concentrations of total P in water or P export per hectare. Although erosion risk was a significant factor influencing the value of these indexes, erosion risk was not correlated with either measure of P loss in these watersheds. Several other watershed characteristics, including average soil test P concentrations, livestock density, proportion of land in annual crops, and the land's inherent capability for agricultural production, were strongly correlated with P concentrations in water (r = 0.80***, r = 0.63**, 0.76***, and -0.70**, respectively). Therefore, these types of P risk indicators require modifications to estimate the risk of P loss under the soil, landscape, and climatic conditions of southern Manitoba.

Abbreviations: BMP, beneficial management practice • IROWC-P, Canada's national indicator of risk of water contamination by phosphorus • SLC, Soil Landscapes of Canada • STP, soil test phosphorus • TP, total phosphorus