Variability of soil water deficiencies for perennial forages in the Canadian prairie region

Abstract

Variability in soil water deficiencies on the Canadian prairies was assessed by calculating forage aridity indices (FAI). These indices integrate climatic and soil parameters with water requirements for optimal forage growth. The model calculated a daily soil water balance by rain entering the soil until full capacity was reached, and evapotranspiration occurring at the potential rate. Supplemental water was added when the available soil water was reduced to 50% of the soil available water-holding capacity (AWC). The summation of these supplements constituted the forage aridity index for the growing season. Daily weather data from 1956–1985, derived for 250 agroecological resource areas (ARA) and 4 different AWCs were input to the model. The results were summarized in terms of fixed probability levels ranging from 5 to 95%. The 50 and 10% probability levels were mapped for the dominant AWC within each ARA. Regression equations were developed to account for local variability in AWC. The FAI during a “normal” year (50% probability) ranged from 66 to 496 mm, while during an ‘abnormally dry’ year (10% probability) it ranged from 180 to 579 mm. Within similar climatic regions, the 50% probability FAI of 100 mm AWC soils was approximately equal to what might be expected once every ten years (10% probability) on a 250 mm AWC soil. Relationships among FAIs at different AWCs and fixed probability levels were developed in order to estimate probability levels between 5 and 95%.