No abstract

Fresh Submarine Groundwater Discharge (FSGD) has been increasingly recognized as a fresh water resource and potential path for pollutants to the ocean. Since directly measuring FSGD requires great effort and is currently only feasible at local scale, we developed the regional transient water balance model Coastal Catchment Regional FSGD (CoCa‐RFSGD) to calculate daily FSGD at long stretches of coastlines. The conceptualization of the model takes into account the main processes between soil and aquifer, and, depending on the research question, in situ data, as well as anthropogenic processes, can be added to the water balance. It provides daily FSGD per coastal catchment. Within this model, FSGD is the precipitation surplus of the catchment area minus surface runoff, which highly correlates with the top soil water‐holding capacity and less with soil hydraulic conductivities. The model uses the island of Java, Indonesia, as an example. Java generates a total FSGD of 15.27 km3/a. Extrapolating the identified correlation of top soil water‐holding capacity and precipitation surplus on FSGD to the world highlights Indonesia, Vietnam, Nigeria, Cameroon, Equatorial Guinea, and Columbia as potential FSGD hot spots. Using mean global aquifer hydraulic conductivities might underestimate the seasonal variation of FSGD on Java. The model can be applied anywhere in the world to locate regional FSGD hot spots without obtaining in situ data, because it can run solely with global data sets. Hence, it is especially feasible for the locations mentioned. Furthermore, FSGD studies can be supplemented with this model by upscaling point measurements of FSGD to a full year.

No abstract

Changing land use in subtropical and tropical catchments to farmland can result in higher nitrogen (N) loss to aquatic ecosystems. Here, we developed a lumped water and N balance model to estimate regional N losses to creeks at catchment scale within understudied subtropical catchments in Australia. The conceptual water balance model CoCa-RFSGD was extended by the nitrogen mass balance in top and subsoil by adding nitrogen cycle transformation estimates depending on meteorological, soil, and land-use properties. The model estimates the impact of pristine and agricultural land use on catchment-wide water quality using only low-order creek samples as water quality measurements of nitrate and nitrite (NO x ) with increased model performance with increased agricultural coverage. The model revealed that an agricultural proportion of 3% in the study site drove a 3.5-fold increase of N losses to creeks and a 6.7-fold increase of N losses to the atmosphere compared to catchments without agriculture. Agricultural land use lost 92 kg-N ha −1 , 85% of which evaded to the atmosphere and 15% was discharged via surface waters. A change from forest to cleared land may increase the total denitrification potential of a catchment. Overall, our lumped model provides a simple but effective tool to upscale local aquatic water quality measurements to the catchment scale, allowing for assessment of changing land use on aquatic N loads in areas with limited data availability.

Submarine groundwater discharge (SGD) flows into coral reefs. In volcanically active areas, the incoming groundwater is typically CO2-rich which can alter the carbon balance and views on how coral reefs function at prevailing high CO2. We quantified dynamic hydrothermal SGD and CO2 fluxes to a Philippine coral reef over a spring-neap tidal cycle. SGD rates, with mean of 35 cm d–1 and 5–95% range of 0–147.8 cm d–1. The groundwater-CO2 fluxes (266 mmol m2 d–1; range: 0–1111 mmol m2 d–1) were up to ∼300-fold larger than evasion of CO2 to the atmosphere. The reef seawater pCO2 (493 μatm; range: 421–680 μatm) remained above atmospheric values and spanned the upper end of the range of atmospheric levels (400–500 μatm) expected for the next century. Because of the hydrothermal SGD, the reef has prevailing above-atmospheric CO2 and is a source to the atmosphere and nearby waters.