Imporlancc o f landscape variables and morphology on nutrients in Missouri reservoirs

Abstract: The proportion of cropland cover in the catchments of Missouri reservoirs, a surrogate for non-point-source nutrient loss from agricultural watersheds, accounts for some 60%-70 % of the cross-system variance in long-term aver-ages of total phosphorus and total nitrogen (n = 126, In transformation for nutrients and logit for cropland). The addi-tion of dam height and an index of flushing rate improved? values to-77 % for both nutrients. Even among reservoir catchments with>80 % grass and forest cover, cropland accounted for most of the variation in nutrients. Reservoir nu-trients showed a strong negative relation to forest cover. Relations between grass cover and nutrients were positive but weak, and grass had no significant statistical effect once cropland was taken into account. Residual analysis suggests that urban reservoirs would have about twice the nutrient level of reservoirs in non-cropland basins (forest and grass). The increase in nutrients with the proportion of cropland and the decrease with forest cover have previously been doc-umented in Missouri streams. R6umB: La proportion des terres agricoles dans les bassins versants des reservoirs du Missouri, une variable de rem-placement pour les pertes diffuses de nutriments dans les bassins versants agricoles, explique environ 60 %- 70 % de la variances des quantites moyennes h long terme de phosphore total et d'azote total dans les bassins (n = 126, trans-formation In des valeurs de nutriments et transformation logit dans le cas des terres agricoles). L'addition de la hauteur des barrages et d'un indice de vidange ameliore les valeurs de? h-77 % pour les deux variables de nutriments