Landscape and Unsaturated Zone Modelling

The Challenge

Groundwater models are important tools that allow scientists to, (i) test hypotheses regarding groundwater system conceptualization and, (ii) utilize model predictions to support decision-making. The efficacy of such models is predicated on their ability to represent hydrogeological structures as well as hydrological boundary conditions. An important boundary condition is at the land surface which is driven by a changing climate. Therefore, it is critical that the migration of water to/from the atmosphere, i.e., groundwater recharge and evapotranspiration, be sufficiently characterised in groundwater models, including flow through the unsaturated zone.

Our Response

WAVES is a point-scale model, designed to simulate water, energy, carbon, and solute balances of a one-dimensional soil-canopy-atmosphere system. It is a process-based model that integrates soil, canopy and atmospheric processes with a consistent level of process detail. WAVES also simulates dynamic interactions and feedbacks between processes. The model is well suited to investigations of hydrological and ecological responses to changes in land management and climatic variation.

Energy Balance

The energy balance calculates net radiation from incoming solar radiation, air temperature and humidity, then partitions it into canopy and soil available energy. Evapotranspiration is calculated using the Penman-Monteith equation using canopy energy, vapour pressure deficit, and air temperature. The Penman-Monteith equation is a ‘big leaf’ model based on the combination of energy balance and aerodynamic principles. The canopy resistance is calculated as a function of net assimilation rate, vapour pressure deficit, and CO₂ concentration. The canopy and atmosphere microclimate including changes in humidity are coupled to handle a multi-layer canopy explicitly.

Carbon Balance

The carbon balance and associated plant growth models calculate actual daily carbon assimilation from plant maximum daily assimilation, and the relative availability of light, water, and nutrients. Limiting interaction effects of temperature on light, and salt in the root zone are modelled explicitly. It is assumed that the actual growth rate is dependent on the potential growth rate and the level of the available resources. To combine the three limiting factors on plant growth into a single scalar we use an integrated rate method, which allows other limiting factors, such as atmospheric CO₂ concentration, to be included. Calculated actual carbon assimilation is used as input to the dynamic allocation of carbon to leaves, stems and roots, and to the calculation of canopy resistance for transpiration.

Soil Water Balance

The soil water balance handles rainfall infiltration, overland flow, soil and plant water extraction, moisture redistribution, lateral flow on soil boundaries, drainage, and groundwater uptake and loss. Soil water movement in both the unsaturated and saturated zones is simulated using a fully-implicit finite-difference numerical solution of a mixed-form of Richards equation. Overland flow can be generated within WAVES from the rainfall rate exceeding the ability of the soil to infiltrate it. A water table may develop anywhere within the soil profile, but particularly where soil layer boundaries provide a high contrast in hydraulic conductivity. If the land surface is sloped then lateral subsurface flow may occur via a perched water table at a soil layer boundary. Flow rate is calculated by Darcy’s law using the positive soil water potential as the thickness of saturated flow.

A regional groundwater depth may be specified, and changed on a daily basis arbitrarily, that interacts with the WAVES soil column. Evaporation and transpiration draw water out of the soil, and when the internal saturated water level is below the regional watertable, discharge into the column occurs and may bring salt with it. Conversely, when the internal water level is above the regional watertable, due to plant inactivity or large amounts or infiltration, water will leak out of the column and will leach salts if they are present.

The Results

WAVES has a history of being used in land use change studies, investigating the impact of land use change on groundwater recharge. These studies have been conducted around the world including Australia, China, Europe and the US. WAVES ability to simulate the carbon balance has seen it used regularly in climate change studies with elevated CO₂ concentrations investigating the impact of climate change on groundwater recharge. These studies include at a continental scale across Australia and at a regional scale in the High Plains in the US. WAVES has also been used to investigate changes in water balance components and vegetation growth due to changes in water table elevation.

WAVES has also been linked to the saturated groundwater flow software, MODFLOW, the worldwide industry standard. This linkage has been applied to the Perth aquifer model known as PRAMS. PRAMS has been an important tool for decision support in Perth regarding groundwater resources management under a changing climate.

WAVES has been a critical element of PRAMS due to its ability to assimilate climate data, particularly forecast data, directly into the groundwater flow simulation. This coupling allows for simultaneous interactions between groundwater flow dynamics and atmospheric conditions to be simulated. This coupling was executed via FORTRAN source code; however, application programming interfaces (APIs) may provide superior flexibility and efficiency for future studies.