Overview of Protocol for Determining Reasonable In- Season Demand & Predicting Demand Shortfall Presented by Mat Weaver Spring 2009

Reasonable In-Season Demand (RISD): the cumulative volume of water projected to be diverted by the surface water user for the entire irrigation season. At the start of season it is always assumed to be equivalent to the Baseline Demand In-season, it is calculated as the cumulative actual crop water need divided by the project efficiency, for the portion of the irrigation season that has already occurred, and the cumulative baseline demand for the remainder of the season. Demand Shortfall: the difference between the RISD and the forecasted supply.

Protocol: Step 1 On April 1st the Bureau of Reclamation and Army Corps of Engineers publishes a joint forecast that predicts an unregulated flow volume at the Heise gage from April 1st to July 31st for the forthcoming water year.

Protocol: Step 2 By April 10th, the Idaho Department of Water Resources (IDWR) will predict and publish a forecast supply for the water year and will compare the forecast supply to the baseline demand (RISD) to determine if a demand shortfall is anticipated for the upcoming irrigation season. Separate forecast supplies, baseline demands, and demand shortfalls will be determined for each member of the Surface Water Coalition (SWC).

Protocol: Step 2 Relationships: Start of Irrigation Season RISD = BD CWN = BCWN Demand Shortfall = BD – FS April Baseline Demand (BD): the sum of the historical volume of water diverted at the head gate and soil moisture adjustment factor for irrigation year Baseline Crop Water Need (BCWN): the average of the total historic volume of crop water need for irrigation year Crop Water Need (CWN): The volume of water required for optimal growth, by all crops supplied with surface water, by the surface water user; it is the product of the area of planted crops and evapotranspiration (ET) less effective precipitation and antecedent soil moisture. Forecast Supply (FS): the combined volume of water available due to anticipated natural flows and total storage (predicted fill and carry over) at the head gate of the surface water user.

Protocol: Step 2

Demand Shortfall: 44,900 ac-ft

Example Twin Falls Canal Company 2002 Irrigation Season ET = (ET 0 *K c *A)-(EP*A) ET 0, Reference ET from ET IDAHO Kc, Crop Coefficient Based on METRIC Data Forecast Supply Historic Natural Flows Historic Storage Volumes

Protocol: Step 3 If the forecast supply is less than reasonable in-season demand, then by definition a demand shortfall exists. IGWA has a responsibility to provide a volume of water to the SWC equal to the amount of the demand shortfall. Two weeks after the day of allocation, IGWA is required to provide evidence, to the satisfaction of the Director, establishing their ability to secure a volume of storage water equal to the entire amount of the predicted demand shortfall. At that time the portion of the demand shortfall equal to the reasonable carryover deficit shall be made available to the SWC. The remainder of the demand shortfall (demand shortfall – reasonable carryover deficit) shall be provided to the SWC at the time of need - typically in September. If IGWA can not meet these requirements by the established due date, IDWR will issue a curtailment order to IGWA for the remainder of the season. As needed the Idaho Ground Water Appropriators (IGWA) will provide and reserve water for the SWC to meet predicted shortages:

Protocol: Step 4 By April 1 st Surface Water Users will provide electronic shape files to IDWR delineating the total irrigated acres within their water delivery boundary. If this information is not provided on time, IDWR will use its own methods to determine the total irrigated acres. Starting at the beginning of April, IDWR will calculate the cumulative evapotranspiration (ET) volume for all land irrigated with surface water within the boundaries of each surface water user. Values of ET will be calculated from LANDSAT 5 imagery utilizing the Normalized Difference Vegetative Index (NDVI) approach for estimating ET. Cumulative in-season ET values will be calculated for each surface water user, approximately once a month.

Protocol: Step 4 Contingency Plan A: Alternative or replacement imagery is obtained by IDWR and utilized to complete the NDVI-ET approach. LANDSAT 7 Other Sources Contingency Plan B: Evapotranspiration is estimated utilizing the ET Idaho approach - a non- imagery based method. Values obtained from ET Idaho will be coupled with crop acreages from the previous year to determine ET volumes for each surface water user.

Protocol: Step 5 Approximately half way through the irrigation season IDWR will (1) evaluate the actual crop water needs of each surface water user up to that point in the irrigation season, and (2) publish a revised forecast supply. This information will be used to: recalculate a reasonable in-season demand adjust the forecasted crop water need adjust demand shortfall.

Protocol: Step 5 Project Efficiency, E p : the ratio of baseline crop water need to baseline demand. Actual Crop Water Need (ACWN): cumulative value of ET volume, for the portion of the irrigation season that has already occurred. Relationships: During Irrigation Season Demand Shortfall = RISD july/sep – FS july/sep

Protocol: Step 5

Adjusted Demand Shortfall: 62,170 ac-ft

Protocol: Step 6 IGWA is required to provide additional evidence, to the satisfaction of the Director, establishing their ability to secure a volume of storage water equal to the revised amount of predicted demand shortfall less reasonable carryover deficit.

Protocol: Step 7 Repeat Step 5 approximately three quarters of the way through the irrigation season. Forecast Supply ACWN RISD Demand Shortfall

Protocol: Step 7

Adjusted Demand Shortfall: 55,603 ac-ft

Protocol: Step 8 For the final time, IGWA is required to provide evidence establishing their ability to secure a volume of storage water equal to the revised amount of predicted demand shortfall less reasonable carryover deficit.

Protocol: Step 9 At the end of the irrigation season IDWR will determine the total actual volumetric demand and total actual crop water need for the entire season. IDWR will evaluate whether predicted shortfalls were adequate and determine final injury, actual carryover, and reasonable carryover.

Protocol: Summary

The End

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