Hydrologists measure and forecast the amount of water in lakes, rivers, and aquifers. Drought takes longer to show up in hydrological systems than in agriculture, especially when reservoirs and rivers are managed to balance the extremes of wet and dry years. Snow is a major component of water supply in the western United States. Snow depth and water content can be measured months before it melts and fills streams and reservoirs.

USGS Drought Watch - Map of below normal 7-day average streamflow compared to historical streamflow for the day of year (United States)