Location: Water Management Research
2013 Annual Report
Using a detailed intra-seasonal model of hydrological and soil processes as a data-generating mechanism results in a realistic inter-seasonal model and leaves open the possibility of analyzing the impacts of different deficit irrigation techniques such as regulated deficit irrigation, sustained deficit irrigation, and partial root zone drying. With that said, the above model, as indicated, uses results from the viticulture literature to generate the necessary intra- and inter-seasonal crop-water-salinity production functions. So, the second major task accomplished this year was to learn and adapt Hydrus-1D to our specific application. Hydrus is a water flow and solute transport software package which will allow us to generate data to estimate, via multivariate regression analysis, yield equations that are a function of irrigation scheme, season, water, nitrogen, and salinity. Unmodified, Hydrus-1D code can consider only water and salinity stresses on the root water and nutrient uptake. A new version of Hydrus-1D, modified by Jiri Šimunek, Hydrus's creator, based on input from a graduate student on this project, can also consider a nutrient stress on root water uptake. The user needs to specify run time (length of a growth season), time steps, soil hydraulic and plant properties, and number of time variable boundary conditions (number of input times, often one or more per day). For each input time, the user specifies precipitation (either actual or irrigation), potential evaporation, potential transpiration, concentration of solute one (salinity) in precipitation/irrigation water, concentration of solute two (nitrogen) in precipitation/irrigation water, and potential solute two (nitrogen) plant uptake. Hydrus-1D sums the potential transpirations to determine total potential transpiration. Similarly, the potential solute uptake is summed to determine cumulative potential nitrogen demand. Combined with soil and crop specific parameters, this data allows Hydrus to estimate relative yield (a value between zero and one, which represents the percentage of potential yield achieved due to considered stresses), which can be used to generate parameters for yield as a function of water, salinity, nitrogen, climate, season, and irrigation scheme. At each time step, Hydrus-1D calculates actual nutrient uptake and actual water uptake, and finds the difference between these values and their potential uptake equivalents. The ratio between these differences and the respective cumulative potential uptakes are compared. The larger value defines the limiting factor and Hydrus-1D considers it to be the instantaneous yield reduction for that time step. The relative yield at the end of the growth season is found by subtracting the summation of all the instantaneous yield reductions from one. So, to summarize, this year we developed and updated the modeling techniques and programs to accomplish this deliverable. Data from the field experiments will be used to calibrate Hydrus-1D, the outcome of which will be data that will be used to update the generalized intra- and inter-seasonal crop-water-salinity production functions to our specific applications.
As specified above, we learned how to use Hydrus-1D and adapt it to our applications so as to generate more accurate crop-water-salinity production functions that will be used to replace the generic functions we’ve used to date to develop and update our intra- and inter-seasonal dynamic grape production model. We also refined our crop-water-salinity production functions to better represent how changes in water application influence within season growth and carryover growth of vine capacity into the next season.
Completion of this research should result in a better understanding of the potential economic impacts of alternative grape management strategies that can be implemented in response to increased water scarcity and salinity on both producer profits and regional economic activity. Furthermore, a better awareness of how the impacts are sensitive to the type of grape industry considered (i.e., table, wine, raisin, juice) and biophysical characteristics of the region will be garnered.