The Role of Drought in the Hydrologic Response of a Managed Coast Redwood Watershed

Hydrologic droughts can be detrimental to aquatic species during low flows in headwater streams and may become more prevalent under drier climates. We used a 55-year record of precipitation and streamflow to explore the relationship between meteorological drought and hydrologic drought in the Caspar Creek Experimental Watersheds in north coastal California. Time series of the standardized precipitation index, the standardized runoff index, and anomalies of precipitation and streamflow were calculated for catchments that were also impacted by harvest treatments at different times in the record. Using the 12 and 24 month SPI time series, we defined severe drought as a period of negative SPI where the drought index value was less than -1.5 for 12 months or more. Two severe drought events were identified occurring 1976 to 1978 and 2013 to 2014. For each drought, the duration, severity, intensity, and persistence were determined. The two droughts were meteorologically comparable, but the more recent 2014 drought was hydrologically more severe than the 1977 drought. We identified an apparent lag in baseflow recovery of up to several years following the cessation of meteorological drought. In addition, we found that hydrologic drought developed in the absence of meteorological drought as a result of an 8-year period of below-normal rainfall. This result suggests even mild multi-year precipitation deficits may propagate to amplified hydrologic extremes. A comparison of precipitation and streamflow anomalies suggests timber harvest may have ameliorated the hydrologic drought in the first few years after harvest and intensified the hydrologic drought during the period of increased transpiration during forest regeneration. Our results provide new insights into streamflow trends following timber harvest and how streamflow, timber harvest, and extreme climatic variation may interact.