SOIL-WATER STORAGE IN DRYLAND CROPPING SYSTEMS - THE SIGNIFICANCE OF CROPPING INTENSIFICATION

Soil and water conservation is essential to the sustainability of Grea t Plains dryland agriculture. We hypothesized that cropping intensific ation improves the efficient use of precipitation. We evaluated long-t erm observations of soil water at three locations in eastern Colorado for a range of pan evaporations (1050-1900 mm), soils, and cropping sy stems, Soils at various locations were mostly of the Argiustoll subgro up except for one Ustochrept and one Haplargid, both at the higher eva poration location, Normal precipitation at the three locations ranges from 400 to 425 mm yr(-1). Systems included a 2-yr winter wheat (Triti cum aestivum L.)-fallow (WF) and more intense 3-yr winter wheat-corn ( Zea mays L.)fallow and winter wheat-sorghum [Sorghum bicolor (L.) Moen ch]-fallow and 4-yr rotations. To quantify the effectiveness of the in tensified systems at utilizing precipitation, we introduce the System- Precipitation-Storage Index (SPSI) and System-Precipitation-Use Index (SPUI), Mean SPSI values were 0.19 and 0.28 for 2- and 3-yr systems, r espectively, meaning that the fallow periods in the 3-yr rotation were collectively 47% more efficient at storing precipitation than fallow in WF, Inclusion of a summer crop, such as corn or sorghum, increased the fraction of precipitation allocated to growing-season crop product ion (i.e., SPUI) from 0.43 in WF to 0.56 (i.e,, an increase of 30%) in 3-yr systems. The gains in efficient use of precipitation with intens ification resulted from (i) reducing the frequency of the inefficient fallow preceding wheat, and (ii) using water for transpiration that,wo uld otherwise be lost during fallow through soil evaporation, runoff, and deep percolation.