Agricultural Land and Water Use Methods in Brief
Agricultural water use depends on a range of factors. These include (1) the land area on which agriculture is practiced, (2) the choice of crops, (3) whether irrigation water rights are held, the availability of water from a given source, and the usage rates of those irrigation water rights. Water is consumed by plants on a daily basis, whether irrigated or not. For rain-fed agriculture crops, water demand will reduce the amount of water in soils, in groundwater reserves, and the amounts seeping to streams; for surface irrigation, the crop water demand will involve diverting water from surface flows when soil moisture is insufficient to meet crop water needs.
The agricultural water use models consist of several interconnected economic models including the limitations imposed by water rights and dynamic models of agricultural evapotranspiration and evolving soil moisture. The models operate at the scale of the Willamette Envision’s map polygons, which are called Integrated Decision Units (IDUs). The four economic models determine: (1) which lands are put to agricultural uses, (2) which crops are grown on a given parcel of land, (3) whether the parcel of land has or will have an irrigation water right, and (4) whether the irrigation water right is used in a given year. These four models interact with the agricultural evapotranspiration model (crop water demand) that simulates daily evapotranspiration (ET) as a dynamic function of climate, land cover, soil water, and growth stage of each crop, from planting date, to ‘greening up,’ to harvest and dormancy. Soil water will vary as a function of precipitation, crop cover, irrigation, and seepage. These four models also interact with water rights that may impose limits on the timing and quantities of water available for irrigation. Farmland transitioned in or out of agriculture versus developed or forest land uses is described in the land use change section. This page provides a brief overview of agricultural modeling in Willamette Envision. For a more complete explanation, refer to refer to Kalinin (2013) and Jaeger et al. (in prep).
In a given year where a particular land parcel is assigned to the agricultural land use, farmer decisions are modeled to simulate crop and irrigation decisions. Irrigation is only possible on IDUs with existing irrigation water rights. These initial decisions are then followed by daily decisions related to planting and harvesting, and (possibly) applying irrigation water. The availability of irrigation water is also subject to regulatory shutoffs in accordance with the prior appropriations seniority system under state law (discussed below).
The combination of decisions, choices, actions, and responses to other factors produces a unique pattern of crop water use, irrigation diversions, soil moisture, and groundwater contributions. It also influences economic returns to farming (annual farmland rent) at the parcel level. To the extent that irrigation water is shut off by regulators, current and expected future annual farmland rent is reduced.
The crop choice model estimates the probability of growing each of seven crop types or groups for the modeled year. The empirical model is estimated at the parcel level based on observed cropping patterns in recent years. The model estimates the crop observed as a function of IDU characteristics including soil quality (land capability class), elevation, and the presence of an irrigation water right, as well as varying attributes, crop prices and expected water availability (for those IDUs with irrigation water rights). Given the estimated probabilities for each IDU, the simulation models determine the crop for each IDU in each year with a random draw reflecting these estimated probabilities. No evidence of crop choices being correlated across years (i.e., a crop rotation schedule) were found in the data or in interviews with farmers or agricultural extension personnel. The resulting modeled values are interpreted as the probabilities for each crop to be grown. For perennial crops (orchards, vineyards, tree crops), a fixed set of IDUs is permanently assigned.
The model of irrigation decisions is based on a detailed farmer survey conducted for WW2100 by the USDA National Agricultural Statistics Service (Kalinin, 2013). Data on a six-year history of irrigation and cropping practices for a sample of fields from 530 randomly selected farmers was collected. From this, an irrigation decision model was estimated to represent the probability of irrigating a specific parcel as a function of parcel attributes (e.g., soil type, elevation), and seasonal factors (e.g., June precipitation).
The economic rent or annual profit from farming a given piece of land can play an important role in farm decisions to plant a crop, irrigate, or transition out of farming. Our estimate of farmland rent takes a “Ricardian” approach that is common in models of the economic returns to agriculture (Mendelsohn et al., 1994). Land value is assumed to equal the net present value of future rents from putting the land to its highest value use; as a result, we expect to see variation in land values and annual rents due to characteristics of the land that would influence agricultural productivity such as soil quality and precipitation or irrigation water rights. Similar to the hedonic model of crop choice, here we decompose the farmland rents associated with factors affecting agricultural productivity (see Kalinin, 2013 for more detail).
Agricultural lands in the WRB that currently do not have irrigation water rights may benefit from opportunities to acquire new water rights under federal contracts for stored water at one of the US Army Corps of Engineers reservoirs. The profitability of a new contract for stored water will depend on a comparison of the irrigation benefits (higher yields and wider range of crop choices) and the additional costs (capital investments in infrastructure, labor, and energy costs). For farmlands with existing irrigation water rights, these costs and benefits are already incorporated into the WW2100 estimates of farmland rent (annual profits) by soil class.
For new contract water rights, we would expect the irrigation premium to be the same as for existing irrigation water rights if the costs of irrigating are similar to the average costs for existing surface and groundwater rights. In the case of new water rights from stored water, we expect the costs to be somewhat higher due to a) the fee paid to the Bureau of Reclamation for the water contract, b) the extra cost for mainline conveyance to bring the water from a below-reservoir tributary to the field, and c) the extra lift required. Whether a new irrigation water right is attractive to a farmer depends on its profitability. Farmlands without irrigation water rights are given the opportunity to acquire new irrigation water rights based on stored water in the “New Irrigation Scenario,” provided that there is an economic justification for doing so.
Water Rights Modeling
Irrigation water demands compete with other water uses, include instream water rights. Because irrigation and instream water rights have the greatest potential to compete directly, we include a description of these water rights and our modeling of them here.
Water is allocated in the WRB according to Oregon water law, which operates according to the “prior appropriations doctrine” used in Oregon and most western states (Getches et al., 2015). The water rights system allocates water according to water right priority date (first date of use historically). Under Oregon law, all water is publicly owned. Water rights certified by the state are defined in terms of the timing of use, the maximum rate of diversion, and the annual volume allowed under the water right. When conflicts arise due to shortage, the more senior water right is given priority, while more junior water rights are required to curtail their water use if it conflicts with the senior water right holder. Water rights may be transferred between points of use under Oregon law when transactions are arranged by parties and approved by the OWRD (Amos, 2008), for example an irrigation right transferred from one farm to another.
Willamette Envision mimics this process: it takes account of the demand or request for water at a given point of diversion (POD) on a given day (from a farm, city, rural residential water user, or instream flow water right), and it evaluates the availability of water from the relevant streams and groundwater source. If there is sufficient water available, it withdraws water to satisfy the demand. If there is insufficient water to meet the needs of an existing water right, the request is denied. At the same time the model determines whether there is a junior water right in the same river reach or any upstream reaches that could be curtailed to make additional water available to satisfy the senior water right. A similar procedure is followed to satisfy instream water rights by protecting flows in streams where such water rights exist. The model includes instream water rights implemented as of 2010. When more than one instream water right applies at the same time to the same reach, the water rights model applies both water requirements. If an instream water right is “senior” to an irrigation water right, the irrigator may be shut off is there is insufficient water to meet both demands. Willamette Envision includes more than 15,000 irrigation water rights, 1,000 municipal water rights, and 90 instream water rights.