Monetizing the role of water in sustaining watershed ecosystem services using a fully integrated subsurface–surface water model

Abstract. Water is essential for all ecosystem services, yet a comprehensive assessment of total (overall) water contributions to ecosystem services production has never been attempted. Quantification of the many ecosystem services impacted by water demands integrated hydrological simulations that implicitly characterize subsurface and surface water exchange. In this study, we use a fully integrated hydrological model—HydroGeoSphere (HGS)—to capture changes in subsurface water, surface water, and evapotranspiration (green water) combined with the economic valuation approach to assess ecosystem services over an 18-year period (2000–2017) in a mixed-use but predominantly agricultural watershed in eastern Ontario, Canada. Using the green water volumes and ecosystem services values as inputs, we calculate the marginal productivity of water, which is $0.45 per m3 (in 2022 Canadian dollars). The valuation results show that maximum green water is used during the dry years, with a value of $1.16 billion during a severe drought that struck in 2012. The average product of water for ecosystem services declines during the dry years. Because subsurface water is a major contributor to the green water supply, it plays a critical role in sustaining ecosystem services during drought conditions. For instance, during the 2012 drought, the subsurface water contribution to green water was estimated at $743 million, making up 72 % of the total value of green water used in that year. Conversely, the surface water contributions in green water provision over the modeling period are comparatively miniscule. This study informs watershed management on the sustainable use of subsurface water during droughts and provides an improved methodology for watershed-based integrated management of ecosystem services.