- Response of agricultural crops to irrigation water deficit is well-understood at the
field scale. Broader scale (watershed and county levels) studies have been less frequently
performed. Data collection at the field level for extensive areas is time consuming and
expensive. The importance of studies at a broader scale for evaluating future scenarios of
agricultural land use justifies exploring alternative approaches that produce reasonable
results in a faster and more economical manner. The objective of this study was to verify
the ability of coarser level data to provide general trends of changes in agricultural
cropping patterns as a function of water use management in the Willamette basin. In this
study, watershed and county level data were used to analyze broader scale crop water
relationships, since these data are abundant and easily accessible. Further, future estimates
of crop water use, based on estimates of yield and acreage changes , were attempted for a
study period spanning current conditions (2000) through 2050.
FAO Yield Response to Water Model was used to relate crop yield and production
to applied irrigation water. Assumptions made to apply this model at the basin level related
mostly to climatic factors and dynamic change of the system. Changes in maximum crop
yield for all important crops grown in the Willamette basin were estimated using a logistic
function parameterized with historic and current datasets. It was important to consider the
dynamic system response when modeling the system to incorporate environmental,
technological and socio-economic factors not reflected in the original model.
The results for crop yield showed a decrease through time for sweet corn, stable
yield for cherries, ryegrass, and grapes, and increasing yield for alfalfa, strawberries and
raspberries. Crops showing higher sensitivity to water deficit included snap beans, winter
and spring wheat, oats and filberts. Production estimates showed 3 groups of crops: predominantly successful (filberts, cherries, ryegrass, oats and spring wheat), stable
(strawberries, alfalfa, orchardgrass, mint and fescue) and predominantly unsuccessful crops
(winter wheat, sweet corn, snap beans and berries).
County level and watershed level data proved helpful in developing a more
comprehensive view of crop production and its irrigation deficit response, using models
incorporating information from finer spatial scale experiments. Socio-economic,
technologic and environmental variables that might influence agriculture over time should
be investigated in further studies.