Base Flow

Like all rivers, the discharge of the Verde river fluctuates rapidly in response to precipitation and snow-melt events. “Baseflow” is the term used to describe flow during periods of low discharge between precipitation events. Baseflow is generally sustained by groundwater inputs along the length of the river and its tributaries and perennially flowing springs are particularly important to Verde flows in the Upper Verde. The full annual cycle of Verde discharge includes periods of generally high flow, with regular inputs of precipitation and runoff, and periods of generally low flow during seasonally dry periods. The month of June generally contains the lowest discharge as June is after temperatures have been warm for several months and after discharge from snowmelt has occurred. However, it is also before the summer monsoon season arrives in July-August. Therefore, this indicator is built from United States Geological Survey (USGS) discharge data from the month of June.

How is it measured?

The average minimum flow was calculated for each 7-day period ending in June (e.g., 7-day low flow) over the entire period of record for five USGS gauges in the watershed. The 7-day low flow was used because it is relatively insensitive to short term peaks in discharge related to precipitation events that are shorter than a week. 

Discharge at each gauge is unique due to its position in the watershed, leading to wide variation in baseflow from the headwaters to Horseshoe Reservoir. Therefore, baseline conditions for each gauge were established separately. An inspection of the data suggested that baseflow has been declining for decades. However, stakeholders stated that if current baseflow could be maintained, conservation efforts would be considered a success. As several of the gauges began declining around 1990, it was decided to calculate a mean and standard deviation of the average June 7-day low flow between 2006 and 2018 and use these statistics as the baseline for each gauge. We then scored each gauge by comparing the average of June mean 7-day low flow over the previous two years to the baseline so that gauges where the 2-year June mean 7-day low flow were above the baseline received a grade above 50% and gauges where the 2-year June mean 7-day low flow was below the baseline mean the zone received a grade below 50%.