Sloping land is somewhat impermeable to water, thus a considerable amount of water is lost by runoff. Crops grown on these sites are less likely to be adversely impacted by saturated soil conditions during prolonged rainy periods, but are prone to a loss of natural fertility due to erosion. Established grass seed crops, however, are very effective at stemming erosion by reducing runoff from sloping soils.
Excess soil moisture can have negative effects on plant growth. However, grasses grown for seed in the Willamette Valley are known to be well adapted to wet soil conditions. In particular, annual and perennial ryegrass will tolerate temporary floods, and do much better than small grains on wet soils. Although short periods of flooding will not damage a stand, extended wet periods will cause stand loss.