|
Consumptive Use and Nebraska Water Transfers
Background In order to comply with the Kansas and Nebraska Republican River Agreement, the State of Nebraska must reduce consumptive use or import water into the Basin. This document reviews the details of how water is currently being used on each acre in the potential area from which enough water could be transferred that would satisfy the requirements of the Agreement.
Goal The goal is to remove no more water from the Basin than is currently being evaporated or transpired by current uses. The formula for this is: Current Consumptive Use minus the Consumptive Use of a profitable non-irrigated crop. The difference is the amount that can safely be transferred out of the Basin without having a net water effect on the source Basin.
Definitions The official quantity of water designated as Consumptive Use is a critical number for many of the water issues. There are two common meanings when talking about Consumptive Use:
Israel Broner, Extension Irrigation Specialist, Dept. of Civil Engineering at Colorado State University says - Crop water use, consumptive use and evapotranspiration (ET), are terms that are used interchangeably to describe the water that is consumed by a crop.
It specifically does not include any consumption from conservation practices such as terraces or minimum till (these are the primary cause of depletion to the stream), nor does it include water used by dry land crops such as wheat or most native vegetation. Nebraska’s version of Consumptive Use is designated as NCU. The difference between the two definitions is important. Any discussion about Consumptive Use needs to be clear about which definition is being used. The primary difference between the two are the details of which things are charged with using water, or what portion of water use is considered a depletion. The official definition is accurate. The Nebraska version creates opportunities and problems.
Consumptive Use Measurement
To measure Consumptive Use, we reference to the National Engineering Handbook published by the USDA / Soil Conservation Service. The chapter on Irrigation Water Requirements is 300 pages and can be found on the web at: http://www.info.usda.gov/CED/ftp/CED/neh15-02.pdf. This gives a detailed description of how to measure water use. It includes a copy of the Blaney-Criddle equation, described in Technical Release No. 21 (SCS 1970), that is used by the Republican River Compact Administration to calculate water use.
Measurement Area Basin Depletion or Consumptive Use varies from location to location and from year to year. We want to know how much water has been historically used by the farmer and what portion of that is removed from the Basin environment. The area of interest is around Holdrege, Nebraska. This is the most economical area to source water for a transfer. Corn is the most common crop in the area. So, our research identifies Consumptive Use for corn in the Holdrege area.
Data Source Much of the work has already been done by High Plains Regional Climate Center. Their web site is http://www.hprcc.unl.edu/online/classic.html. They maintain daily historical climate data. This data can be accessed for a fee. The HPRCC calculates evapotranspiration for the crop of choice emerging from the ground on the date of choice and with the number of growing degree days of choice. The HPRCC computes the first several steps laid out by the National Engineering Handbook. For our purposes, we access the daily evapotranspiration (ET) and precipitation records for Holdrege, 2850 growing degree day corn (same period the DNR uses in its report to the Legislature), emerging on May 1. The HPRCC adjusts the ET for the crop coefficient.
Consumptive Use at Holdrege Note that Evapotranspiration (ET) and Consumptive Use (CU) are synonymous. We collected ET records for 10 years – from 1996 through 2005. Corn ET ranges from 29.0 to 29.1 inches per acre. This means one acre of corn in the Holdrege area transpires between 29.0 to 29.1 inches of water during its growing season. This water leaves the plant and is evaporated into the air. The wind blows this away, and the water is lost to the Basin. This water was used by each acre of irrigated corn. The water used by the corn is currently legal, acceptable, and a desirable activity. Whether this water comes from precipitation or irrigation or some combination is irrelevant; it is still removed from the environment. Corn will use 29.1 inches of water, if it is available. This is the most it will extract from the environment, assuming a standard seeding rate. If more water is available, the plant ignores it and the water either runs off into the stream or seeps past the root zone into the aquifer. This means that allocations restricting irrigation are irrelevant if precipitation plus irrigation are greater than the consumptive use requirements of the plant.
Irrigation Requirement The amount of precipitation varies from year to year. If the precipitation arrives out of season, then it either evaporates or seeps into the soil. Different soils have different moisture-holding capacities. A common soil type in the source area is Peoria Loess. Its moisture-holding capacity is about 1.75 inches per foot. If the precipitation falls at a rate faster than the plant can use it, then the water either runs off to the stream or seeps past the root zone. Formulas have been developed to estimate the portion of the precipitation that is available to the plant based on the crop stage, climate conditions, and precipitation rates. The result of processing the data through these formulas yields the Effective Precipitation. This is the amount of precipitation available to the crop. It is always equal to or less than the actual precipitation rate. We again reference the National Engineering Handbook published by the USDA / Soil Conservation Service. It can be found on the web at: http://www.info.usda.gov/CED/ftp/CED/neh15-02.pdf. It provides the following flow chart, detailing how to compute irrigation requirements:
The formula is Consumptive Use minus Effective Precipitation divided by the inefficiencies of the irrigation delivery system. For example, 29.1 inches ET minus 14.1 inches Effective Precipitation minus Soil Profile Storage of 3.3 equals 11.70 inches Net Irrigation Requirement. Divide this number by the efficiency of the irrigation delivery system (80% yields 15.0 inches required irrigation amount). Providing less than this at the right time will result in yield losses. Remember, these are the averages, and the actual annual numbers can be significantly different. The actual range is between 0 and 22.9 inches. (ET – EP - SPS) / IE = Applied Irrigation Required ET = Evapotranspiration EP = Effective Precipitation SPS = Soil Profile Storage IE = Irrigation Efficiency ET is nearly constant year to year. For Holdrege, it averages 29.1 inches. EP has ranged from 6.7 to 33.4 during 1982 through 2005. The average is 14.1 inches. SPS varies by soil type. During droughts, the soil profile contains less water. We applied all off-season precipitation to the soil profile until the profile was full. Over the last ten years, the soil profile has been fully recharged twice. We used a three-foot profile with 1.75 maximum inches per foot. Most of this profile is unavailable to the plant until its roots are sufficiently developed. The average is 3.3 inches. IE is dependent on two primary factors: Gravity vs center pivot - A center pivot with the latest water saving innovations can achieve efficiencies of up to 90%. Gated pipe or ditch delivery efficiency rates are dependent on the length of the rows, but is about 50% to 70% efficient. Timing - Applying an inch of irrigation water and then receiving an unanticipated 2 inch rain means the one inch of irrigation was “wasted” or inefficient. Assuming an efficiency of 80% during 1996 though 2005, the irrigation requirement for the Holdrege area ranged between 0.0 and 22.0 inches. The twenty-three year average irrigation requirement is 15.0 inches. Over the last nine years, the average irrigation requirement has been 17.4 inches. During the last nine years, growing season precipitation has been below average 8 out of 9 years. This dry period has depleted moisture from the soil profile and increased the irrigation requirements. ET/Precipitation/Irrigation Table
It is important to note that the amount of water used by the corn crop, also known as consumptive use or evapotranspiration, varies little from year to year. According to the High Plains Regional Climate Center, it remains constant in the Holdrege area at 29.1 inches per acre. The irrigation requirement is not related to the total needs of the crop. However, if irrigation water is not available to supplement the effective precipitation, then the consumptive use will be less, as will be the yield.
Water Available To Transfer The amount of water that can be transferred out of the Basin without having an effect on the source Basin is the difference between the current or historical consumptive use and the consumptive use of a replacement crop. Note that the irrigation requirements of any particular crop are not relevant. Their total consumptive use is. If the current use is 29.1 inches and a non-irrigated crop uses 19.0 inches, then the amount that can be transferred without having an effect on the source Basin is 10.1 inches.
Replacement Crops The Consumptive Use of a non-irrigated crop can never be more than the Effective Precipitation plus the Soil Profile Storage. If the water is not present, the plant cannot transpire it. We assume the replacement crop will use all available precipitation and all water out of the Soil Profile Storage up to the maximum ET for that crop and seeding rate. However, if a farmer knows that he will not irrigate the crop, he will prudently plant at a lower seeding rate. Thus, his maximum ET will be lower than if he had seeded at the full rate. As part of the contract signed with the farmer for the purchase or lease of the water, the dry land seeding rate can be set so the maximum ET for dryland corn would be no more than 19.5 inches. The following table and graph assume the replacement crop ET has been limited using this method.
As the table shows, if the maximum ET for the replacement crop is 19.5 inches, then the minimum amount that would be safe to transfer would be 9.6 inches. The average is 13.1. The formula assumes that the dry crop will use all available water, including what is in the soil profile, up to the maximum ET for the specified crop. If a non-irrigated replacement crop can be found that uses less water, then more water can be transferred. The reason for wanting to maximize the amount of water transferred per acre is to minimize the cost. All irrigation rights would be leased from the landowner at above-market rates. The fewer acres that need to be purchased to obtain the water needed, the less the expense. If groundwater is used as the source water, then the cost of building the infrastructure to move water from the source well to the transfer point can be minimized by reducing the number of source points. Another way to maximize the amount of water that can be transferred from any one acre is to lease acres that have historically raised crops with ET rates higher than the corn that we have studied here. As the graph shows, this would be any field raising alfalfa or irrigated grass. As a side note, the retirement of fields that would reduce consumptive use the most are alfalfa and grass fields. Yet, they are prohibited from participating in CREP programs, which are designed to minimize consumptive use.
|
|||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
