A fourth objective in soil water conservation for the dryer, summer fallow areas is to maintain adequate water in the seed zone for crop establishment. Although water losses from runoff and deep percolation can be highly significant in some years and locations in the crop-fallow areas, losses normally comprise a relatively small percentage of the total precipitation. Where these losses are a problem, yield potential can be dramatically reduced. Transpiration losses of water by weeds can also be significant, but weeds can usually be controlled with tillage or herbicides. Evaporation usually accounts for the bulk of the water loss during fallow.
Evaporation is also generally the most difficult water-loss process to control.
Evaporation Processes Papendick explains that there are two main stages in the soil-water evaporation process. The first or constant-rate stage occurs when the soil surface is wet. The evaporation rate is at a maximum and is controlled by the evaporative demand of the atmosphere and soil surface conditions. Surface residue level, roughness and other surface conditions can modify the influence of atmospheric conditions by reflecting heat and slowing air movement near the soil surface. On a bare soil, the first stage evaporative rate can approach that from an open pan of water.