A coastwide perspective on defining and characterizing drought events in Texas estuaries

Background/Question/Methods

Texas estuaries are vulnerable to natural reductions in freshwater inflow now more than ever due to increasing demands for freshwater resources that have the potential to induce drought-like inflows at intervals more frequent than experienced by natural droughts.  However, methods to identify drought effects in estuaries commonly are defined by conditions occurring within contributing watersheds as opposed to conditions occurring within the estuary itself.  The focus of this study was to employ a method that allows conditions specific to an estuary (i.e.,surface inflow) to define a period of drought.  Using a modified approach from Ward (2010), residual mass curves depicting cumulative inflow deficits were calculated for inflow records spanning up to 73 years to identify historical periods of drought in ten Texas estuaries.  Droughts were defined as periods in which monthly inflow was below a specified drought criterion of 60 percent less than long-term mean monthly inflow, for at least a minimum of one year.  This presentation will describe drought characteristics and frequency among Texas estuaries as determined by the cumulative inflow departure method using drought metrics such as intensity, severity, and duration to compare drought events across estuaries, and also will include an evaluation of corresponding salinity conditions.

Results/Conclusions

Texas estuaries experienced between ten and 15 drought events over the 73 year period, and multi-year drought periods were common.  In all ten estuaries, the cumulative inflow departure method identified either the drought of record (during the 1950’s for most of Texas) or the recent, ongoing drought as one of the longest, most severe, or most intense.  Surprisingly, upper coast estuaries that receive high annual inflow volumes recorded just as many drought events as the more arid lower-coast estuaries, although events on the upper coast tended to be of shorter duration.  Upper-coast systems accumulated large deficits, upwards of 40 million acre-feet, during long drought events; whereas, the arid lower-coast estuaries accumulated less inflow deficit and also experienced less intense and less severe droughts.  Increased salinity condition ranging from three to ten additional practical salinity units was observed during drought periods in all estuaries.  This analysis demonstrated that drought is a frequent factor affecting estuarine health and productivity in Texas, but also that the cumulative inflow departure method identified periods in which estuaries experienced low inflow conditions that may not be captured by more typically reported terrestrial drought indices.