Location: Water Management Research2013 Annual Report
1a. Objectives (from AD-416):
Develop an intra-seasonal model of grape production that evaluates the economic impacts of different leaching and deficit irrigation strategies on grape production within a single season and across seasons. Analyze the economic impact of reduced water suplies and increased salinity on field and farm-level management decisions.
1b. Approach (from AD-416):
The research will use the results of a series of field studies on cooperating farms, laboratory studies at UC Davis, greenhouse studies and sand tank studies at U.S. Salinity Laboratory in the model development.
3. Progress Report:
This project contributes to meeting objective 2 “Develop sustainable water management strategies for perennial horticultural crops with limited and impaired water supplies” of the parent project. Sustainable water management strategies require both analysis of the physical system and the economic system. If the system is not economic for the grower then it is not sustainable. This project provides a detailed economic analysis of the proposed water management strategies both at the farm and regional scales. Two main tasks were accomplished this year. A field-level, bio-economic model was developed that is capable of describing the inter-seasonal dynamics of water applications to perennial crops. The model incorporates the effects of crop age, soil salinity, and irrigation history on yield potential by using an unobserved biomass state variable. The biomass variable, which represents vine capacity, captures the irrigation history of the crop in a single state variable, thus allowing the use a stochastic dynamic programming framework to analyze optimal management decisions over the life of the crop given stochastic water supplies. Yields are a function of plant age, biomass, current season water applications and salinity. The biomass law of motion is a stylized representation of findings in the viticultural science literature and is calibrated to reproduce observed yield effects, both within season and across seasons, from varying irrigation quantities. Salinity affects yields in the current season via water uptake as well as in the future by changing root zone salinity levels. While the focus of the study is on the inter-seasonal effects of varying seasonal water applications due to scarce and variable water supplies over time, some consideration of the intra-seasonal dynamics is required in order to create a realistic and flexible model of irrigated wine grape production. Since several factors affect how water applications translate into water uptake by grapevines, it is more accurate to develop a model that reflects the changing agronomic conditions throughout the growing season rather than treating the seasonal irrigation water as a single quantity of water applied once per season. Plant transpiration depends on the soil moisture and salinity which are constantly changing over the course of the season. Depending on soil conditions, only a portion of the water applied at any point in time will become available to the grapevines as some water will inevitably be lost to deep percolation into the water table, evaporation from the soil surface, or run-off into other bodies of water. These processes are inherently non-linear and have thresholds, which imply that models which depend solely on seasonal averages may give misleading results. 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.