Quantify average water flow rates into/out of the watershed and create a water budget
Water level and salinity at all three stations were affected by both tides and precipitation events (Fig. 2). During the entire record, four tropical events passed by the area. However, during the more limited period of the water budget (7/4/2020–9/19/2020), only two tropical cyclones passed by the area, Hurricane Hanna and Hurricane Laura, and the precipitation observed during this period was 24 cm (9.5 in.) Precipitation throughout 2020 (32.36 cm) was 5% lower than the average rainfall from 1954 to 2020 (34.12 cm).
The UB station was most responsive to precipitation as shown with its brief peaks following precipitation events, followed by a generally rapid return to its baseline. At the LB station, water level is largely driven by the tides and storm surges. Precipitation can also cause water levels at LB to rise, just slightly below those at UB (1.4 m at UB compared to 1.3 m at LB on September 22, 2020). The Chinquapin Road station was the most unique of the three. At times, the Chinquapin Road station appeared to respond differently to precipitation events, and the water levels generally took longer to decrease after these events, suggesting that it was responding to different inflow sources (i.e., Lake Austin). Further, the daily tidal range at Chinquapin Road was 4 cm, the lowest of the three stations, potentially indicating a hydrologically isolated location.
The ADCPs provided stream flow volume and direction (Fig. 3). When coupled with water elevation data from the accompanying CTD, we obtained a more complete understanding of flows at the UB and LB stations. The volume of flow between UB and LB can differ drastically. At the peak of Hurricane Hanna (July 25, 2020), almost 65,000 m3/hour was flowing at LB. In contrast, the peak upstream flow at UB was only 19,000 m3/hour. The flow volumes also varied drastically during regular tidal periods, with a difference of almost 5,000 m3/hour between the incoming and outgoing tides and at LB, and a difference of only 200 m3/hour at UB. As shown in Fig. 3b, the downstream flows at LB were not in balance with upstream flows. This supports the notion that there were alternate outlets for LB flow exiting into the GIWW (other than Big Boggy Creek).
At the UB station, the total upstream flow during the budgeted period of 7/4/2020 to 9/19/2020 was 1,390 ML (Fig. 4) consisting of incoming tides. Total downstream flow measured 2,120 ML, consisting of outgoing tides and freshwater flows. The imbalance between upstream and downstream flows was 730 ML and represented the freshwater inflow quantity. It thus represented the difference between the total precipitation and evaporation in the watershed further upstream, excepting any unbudgeted losses or gains (see Methods).
The total watershed area upstream of the UB station was 7,927 hectares. The quantity of precipitation multiplied by this area resulted in a far higher value than the 730 ML observed inflow volume. Thus, the effective watershed area was calculated as 225 hectares, which was only 2.84% of the total watershed area (Fig. 5). This quantity matched what could be expected given direct capture of precipitation into the system. In other words, our sensor stations may have only observed precipitation that fell into open water bodies, low marsh, and high marsh areas, insinuating that overland flows did not play as large a role as previously assumed. Low topographical relief in the study area may explain this phenomenon. Flat upland areas may not readily flow into the waterways and instead be subject to high rates of evaporation.
At the LB station, the total upstream flow during the study period was 6,652 ML, consisting of incoming tides and storm surges. Total downstream flow measured 4,220 ML, consisting of outgoing tides and freshwater flows. The difference between upstream and downstream flows was 2,431 ML. Precipitation was estimated as 4,961 ML.
We found that the total watershed area upstream of the LB station—and downstream of UB—was 2,780 hectares (Fig. 5). The effective watershed area was calculated as 1,532 hectares, which was 55% percent of the total potential area. This percent was much higher compared to UB, because much of the LB watershed was effectively directly capturing the precipitation in open water, low marsh, or high marsh areas.
It is important to note that there was a large imbalance in the water budget at LB. There was approximately 7,392 ML unaccounted for as calculated by the effective watershed. This imbalance value includes upstream flows as well as precipitation. When taking into consideration the downstream flows from UB into LB (2,120 ML), there was a total of 9,502 ML. This large surplus of water suggests that there were other significant outlets in the marsh complex (Fig. 6). These outlets likely only connect and move water out of the marsh complex and into the GIWW when water levels exceeded 0.45 m (NAVD88). The only alternate outlet that may not be water-level dependent was the culvert beneath Chinquapin Road. Groundwater flux may account for some of this loss, however it likely does not account for much of the volume as we would generally not expect tidal waters to flow into groundwater in this coastal area due to hydraulic head pressure.
Hindcasted and forecasted inflow volumes
We found that the average historic inflows at UB and LB were 255 Megaliters (ML) per month and 1,744 ML per month (in July, August, and September), respectively. Hindcasted inflows followed a positive mean trend, increasing over this budgeted time period from 1954 to 2019 (Fig. 7). Note that for each historic year only the three summer months were used to match the study period. Over the historic period, nine years were above the positive RMSE threshold and seven years were below the negative RMSE threshold. Of the three climate change scenarios through 2100 that were explored, the best-case scenario was the B1 scenario, in which a 4% increase in inflow was predicted. The intermediate scenario, A1B, estimated a 3% increase in inflow. Finally, A2 yielded the worst-case prediction, where inflows were predicted to decrease by 4%. It is crucial to note that these were just predicted changes in the mean trend of inflow. Years with inflows higher or lower than the mean and outside of the RMSE bounds will occur, and these are more likely to alter ecosystem function than a change in the mean trend.