Groundwater pumping, artificial recharge, precipitation, and temperature all affect groundwater. Because the base flow of the Verde River originates as groundwater, all those factors combine to influence the flow of the upper Verde River. USGS data shows a steady decline in base flow.
Verde River: Lowest 7-day annual flows switch from gaining to losing in mid-1990s
The U.S. Geological Survey (USGS) Paulden and Clarkdale streamgages have operated since the mid 1960s on the Verde River at river miles 10 and 39, respectively. Every 15 minutes they transmit streamflow data from which the flow rate in cubic feet per second (cfs) is calculated. A cubic foot of water equals 7.48 gallons.
These gages are a valuable adjunct to the much briefer gage records in the PrAMA. The large multiday winter storms that are critical sources of groundwater recharge are regional, simultaneously adding groundwater and affecting streamflow in the Verde Valley and the Prescott area. Individual monsoon-season storms tend to be brief and local, but the seasonal character of monsoon precipitation, abundant or sparse, tends to be regional.
The Verde flows year-round without interruption. The lowest flows each year normally follow extended periods in which no runoff from rain or snowmelt reaches the river. Each blue dot (above) represents the average of the seven consecutive days of lowest measured flow in each year. We take these lowest flows as an approximation of the contribution of groundwater to the river. The red circles mark exceptionally low streamflow known to be coincident with documented massive anomalous groundwater pumping in the Paulden area and are omitted from our calculations from the Paulden gage record.
The lowest-flow patterns through time in these graphs are strikingly similar in shape even though the numeric values recorded at the Clarkdale streamgage are roughly 3 to 4 times the magnitude of their counterparts at the Paulden streamgage. Each gage recorded approximately 30 years of gradually but irregularly increasing lowest flows ending in the mid-1990s with an abrupt switch to a pattern of generally decreasing lowest flow values. The change to decreasing lowest flows is essentially coincident with the previously noted increased rate of accumulating overdraft in the PrAMA. Further it implies that the groundwater that enables year-round flow of the Verde River is diminishing.
