Diurnal Cycles in River Discharge: a Key to Understanding Snowmelt, Evapotranspiration, and Infiltration

The United States Geological Survey (USGS) makes hourly measurements of river discharge every day at thousands of stream-gauge stations across the United States. Regular patterns of diurnal variation have been observed in hundreds of these rivers, many of which are primary water sources for cities, industries, and agriculture. Diurnal cycles are evident in many river records (Figure 1, Merced River discharge), and often the diurnal amplitude, measured as half the difference between the daily maximum and minimum discharge, exceeds 10% of the daily mean flow. These patterns are ultimately caused by diurnal variations of solar radiation and temperature, which regulate the amount of water added to or removed from the rivers each day. Processes identified as producing diurnal streamflow cycles include daily variations in evapotranspiration, infiltration, and snowmelt. These diurnal cycles relate directly to the processes contributing to the flow of each river and can be used to enhance the ability to predict how that flow will evolve over longer time scales.