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'Data & Modeling - Connected to - Policy & Law: Is it a Dovetail or Ruffled Feathers? '
Gary Spackman (gary.spackman@idwr.idaho.gov), ID Dept of Water Resources

Idaho law requires that the director of the Idaho Department of Water Resources conjunctively administer the delivery of hydraulically connected ground water and surface water. The water rights are administered pursuant to the prior appropriation doctrine. The prior appropriation doctrine was established for the delivery of surface water in times of shortage. The doctrine presumes a real-time ability to determine how much water is available, to compare the flow to existing water rights authorizing diversion of available water, and to curtail junior priority water rights if there is insufficient water to satisfy senior priority water rights. The time and quantity relationships between ground water and surface water are much more uncertain. As a result, the relationships must be modeled, and model simulations must be the basis for administration. In Idaho, holders of senior priority surface water rights authorizing large diversions from the Snake River derive their water supply from (1) natural flow in the river; (2) springs that discharge into the river; and (3)reservoirs operated by the US Bureau of Reclamation. Many factors affect spring flows to the river. Some of these include reduced recharge because of improved irrigation practices and ground water pumping by the holders of junior priority ground water rights. The courts have held that the director must predict, before the irrigation season begins, what the water supply will be for the holders of the senior surface water rights. The director must then determine how much will be needed for the lands irrigated with the surface water, and whether there will be sufficient supply for the the surface water irrigation. If not, the director, must require mitigation from the holders of junior ground water rights or curtail the ground water diversions. Predicting a water supply several months ahead of the irrigation season is challenging. In addition, predicting the water need is difficult. Finally, ground water model simulations determine which water rights should be curtailed to provide benefits to the senior priority surface water rights. The overlay of the predictive modeling with the policy/legal requirements is not always smoothly layered. My presentation will discuss the difficulties encountered when the predictions are imperfect and the director must make discretionary policy decisions that may conflict with differing interpretations of scientific information and interpretations of the law.