Water-budget accounting for tropical regions model (WATRMod) documentation

Abstract

The water-budget code that is described in this document is a modified version of the code that has evolved over time to estimate groundwater recharge for Pacific islands. The author of this report developed the original version of the code, which John A. Engott of the U.S. Geological Survey improved over time. John A. Engott's latest version of the code served as a starting point for the code described in this document. Adam G. Johnson and Alan Mair of the U.S. Geological Survey helped by suggesting conceptual improvements to the code and by testing the code.

Identifying the relative importance of water-budget information needed to quantify how land-cover change affects recharge, Hawaiian Islands

“…WATRMod uses the equations and information from model input files to simulate hydrologic processes that affect recharge, including rainfall, fog interception, irrigation, septic-system leachate, runoff, and evapotranspiration. For the purpose of this report, "model input" includes any information in the model input files, whose format and content requirements are described by Oki (2022a). Model input includes (1) data, such as rainfall data and land-cover information for the area being modeled, (2) parameter values, which are numerical values assigned to parameters, such as root depth, in the model equations (Finch and Aronson, 1982), and (3) initial conditions at the start of a model simulation.…”

“…WATRMod is a modified version of older models that were used to estimate spatially distributed mean annual recharge for several Hawaiian Islands for historical and recent climate conditions (Engott, 2011;Engott and others, 2017;Izuka and others, 2018;Johnson and others, 2018;Oki and others, 2020), for potential future climate conditions (Mair and others, 2019), and for potential future land-cover conditions (Brewington and others, 2019). Compared with older versions of the model used by the listed studies, WATRMod uses a similar water-budget approach and requires most of the same types of model input, but it uses an updated source code (Oki, 2022b) and contains more options for users (Oki, 2022a).…”

“…The purpose of this report is to describe a study that identified and summarized the types of hydrologic information that are critical for quantifying how recharge for the Hawaiian Islands might be affected by managed or unmanaged watersheds. The study used WATRMod (Oki, 2022a), which was designed to estimate spatially distributed recharge and other water-budget components for tropical islands. The study completed a sensitivity analysis of WATRMod to evaluate the sensitivity of modeled recharge estimates to seven parameters that might be affected by land-cover changes within a watershed.…”

“…Information used to prepare these model-input components for the sensitivity analysis is described in this report (see "Model Input Not Evaluated" section). Oki (2022a) provided descriptions of all model input required for the model.…”

“…The modeled sensitivity scenarios included those that tested fog-catch efficiency values of 1.2, 1.6, and 2 for forest landcover types. Scenarios that tested fog-catch efficiency values that exceeded the upper end of the range (0 to 1) described in WATRMod's documentation (Oki, 2022a) were included and assumed to represent plausible conditions because the spatial distribution of fog interception for the Hawaiian Islands is uncertain. Furthermore, the baseline area-weighted average foginterception estimates for windward forests and leeward forests were conservative relative to fog-interception estimates that were derived from field measurements at multiple forested locations in the Hawaiian Islands (see, for example, DeLay and Giambelluca, 2010; Giambelluca and others, 2011;Juvik and others, 2011;Takahashi and others, 2011).…”