Verde Valley AZ (November 20, 2012) – The Verde River Basin Partnership believes that the Verde River is a priceless asset for the communities of central and northern Arizona. Its streamflow provides irrigation in the Verde Valley and drinking water for the Phoenix area. The river and its perennial tributaries are the lifeblood of agriculture and rural/suburban lifestyles in and around the Verde Valley. The river fuels a recreation industry; provides crucial habitat, serves as a lifeline for a wide variety of wildlife and vegetation, and this very habitat plays a role in drawing people to the river for recreation.
About half of the Verde River’s annual flow is provided by discharge to the river of groundwater from the same aquifers that supply virtually all of the household water for residents of the upper and middle Verde River watersheds, which include the Prescott area, Big Chino and Williamson Valleys, and all the communities of the Verde Valley. Thus, the groundwater and surface water are, in fact, a single resource. It is that connection that sustains the wonderful Verde River year-round. Unfortunately, that same connection of groundwater and the river, in combination with the necessity of water for an ever-expanding population, threatens the river’s year-round flow and the rich natural habitat, wildlife, and human activity that it supports.
A simple concept underlies the concern: Groundwater pumping reduces the amount of groundwater that flows to streams, and, in some cases, can draw streamflow into the underlying groundwater system. The concept has been understood for more than 70 years, and is explained specifically for the Verde Valley in a recent U.S. Geological Survey (USGS) report (http://pubs.usgs.gov/sir/2010/5147/sir2010-5147.pdf). Pumped long enough, a well eventually draws its full supply of pumped water from connected surface-water features—streams, springs, wetlands, and riparian zones—that are supplied by groundwater.
Knowing how to mitigate or prevent such loss of Verde River streamflow owing to groundwater pumping requires understanding the cause, potential mitigating or preventive water-management strategies, and the likely magnitude and timing of streamflow depletion. Until recently, some of these critical factors—especially the magnitude and timing of streamflow depletion—were unknowable. However, there is now a powerful scientific tool, the Northern Arizona Regional Groundwater-Flow Model (NARGFM), to guide critically-important water-management policy and decisions.
The NARGFM (see http://pubs.usgs.gov/sir/2010/5180/) was released for public use by the USGS in April 2011—more than 1½ years ago. It draws extensively on regional hydrologic studies carried out beginning in 1999 in northern and central Arizona by the USGS in cooperation with the Arizona Department of Water Resources. Thus, publication of the NARGFM culminates more than a decade of tax-supported hydrologic research and reporting in north-central Arizona, representing a multimillion-dollar expenditure by all the cooperating parties.
The NARGFM was developed to help assess the adequacy of the regional groundwater supply and the potential effects of past, present, and future groundwater use on water levels, streamflow, and riparian vegetation. Water-resource management was recognized to be of particular interest for water-management decisions in the Little Chino, Big Chino, and Verde Valleys. In these areas, discharge of groundwater to the Verde River and potential capture of groundwater flow from adjacent areas including the Coconino Plateau and Little Colorado River basins are of primary interest.
While development of the NARGFM by the USGS was under way, the Yavapai County Water Advisory Committee completed a set of water-management scenarios that address elegantly planned future groundwater-pumping requirements in the upper and middle Verde River watersheds. A commitment was made to have the USGS use the NARGFM to examine the modeled effects of the scenarios on such factors as groundwater levels and streamflow in these watersheds. From the standpoint of water-management policy to protect the flow of the Verde River over the long term, it is imperative that all use the model now to evaluate likely impacts on the river from pumping in both the upper and middle Verde River watersheds.
The NARGFM report notes that “The model can be used by resource managers to examine the hydrologic consequences of various groundwater development and climate-change scenarios for regions that are sub-basin or larger in area. Use of the model for site-scale investigations may require additional data to better define the local hydrogeology.” A series of experiments conducted by the USGS in recent months confirms that the NARGFM is indeed appropriate to guide resource managers in addressing questions for sub-basin or larger areas. For example: What, approximately, are the likely effects on Verde River streamflow from groundwater development in the Verde Valley sub-basin and the adjacent basins or sub-basins? Has such groundwater development already affected Verde River streamflow? Are the effects likely to develop in years, decades, centuries, or millennia?
We can’t wait. Groundwater flows continually through the aquifers, but the flow rate is slow. Thus, the effects of groundwater pumping far from the river can take many years to become obvious. By the time the effects are fully obvious, the damage will have been done to the Verde just as it has already been done to other Arizona rivers. The powerful science-based tool represented by the NARGFM must be put to use now!
-Edward W. Wolfe, Ph.D., retired USGS geologist, former chair of the Verde Watershed Association and the Verde River Basin Partnership, chair of the Verde River Basin Partnership Technical Advisory Committee.