Brook is a hydrological model that simulates snow accumulation, soil water and streamflow from daily precipitation and temperature. The original version of this model was developed in the late 1970s (Federer and Lash, 1978). BROOK2 is a newer version used extensively for teaching and research of snow-soil-streamflow dynamics.
Brook is a deterministic model that looks at hydrologic dynamics at the watershed level. The model contains several water storage compartments: snow, root zone, unsaturated zone below the root zone and groundwater. Input of the model is daily mean temperatures and precipitation. Snowmelt by degree-day factor is adjusted for by leaf area index (LAI) and stem area index (SAI).
BROOK90 contains major modifications to the state equations in the model. For example, potential transpiration and soil evaporation are calculated using the Shuttleworth-Wallace equations and vertical water movement is calculated using a modified Clapp-Hornberger relation. Input required for BROOK90 differs slightly form the earlier versions. Weather information for example must include daily maximum and minimum temperatures, and wind speed. Several additional parameters are needed, including: soils water properties, and stomatal response parameters.
BROOK90 simulates the land phase of the precipitation evaporation streamflow part of the hydrologic cycle for a point or for a small, uniform (lumped parameter) watershed. There is no provision for spatial distribution of parameters in the horizontal. There is no provision for lateral transfer of water to adjacent downslope areas. Instead, BROOK90 concentrates on detailed simulation of evaporation processes, on vertical water flow, and of local generation of stormflow. Below ground, the model includes one to many soil layers, which may have differing physical properties.
BROOK90 has been designed to be applicable to any land surface. The model has numerous parameters, but all parameters are provided externally, are physically meaningful, and have default values. Parameter fitting is not necessary to obtain reasonable results. However, a procedure is described for modifying important parameters to improve the fit of simulated to measured streamflow.
BROOK90 is designed to fill a wide range of needs: as a research tool to study the water budget and water movement on small plots, as a teaching tool for evaporation and soil water processes, as a water budget model for land managers and for predicting climate change effects, and as a fairly complex water budget model against which simpler models can be tested.
Evaporation has five components: evaporation of intercepted rain (IRVP), evaporation of intercepted snow (ISVP), evaporation from snow (SNVP), soil evaporation (SLVP) from the top soil layer, and transpiration (TRANI) from each soil layer that contains roots. Potential evaporation rates are obtained using the Shuttleworth and Wallace (1985) modification of the Penman-Monteith approach. Evaporation of intercepted rain or snow is calculated with a canopy resistance of zero and aerodynamic resistances based on canopy height, coupled with a canopy capacity and an average storm duration. For potential transpiration, canopy resistance depends on maximum leaf conductance, reduced for humidity, temperature, and light penetration. Aerodynamic resistances are modified from Shuttleworth and Gurney (1990); they depend on leaf area index (LAI), which can vary seasonally, and on canopy height, which determines stem area index (SAI). Soil evaporation resistance depends on soil water potential in the top soil layer. Actual transpiration is the lesser of potential transpiration and a soil water supply rate determined by the resistance to liquid water flow in the plants and on root distribution and soil water potential in the soil layers.
Snowmelt is based on a degree day factor and accounts for snowpack temperature and liquid water content. The factor is modified for canopy cover as determined by LAI and SAI. Snow evaporation or condensation depends on the aerodynamic resistances and the vapor gradient; however, an arbitrary reduction factor is required.
Net throughfall plus snowmelt may 1) infiltrate into the soil matrix of the surface horizon (INFLI(1)), 2) infiltrate directly to deeper horizons via vertical macropore flow (INFLI), 3) go immediately to streamflow via vertical macropore flow followed by downslope pipe flow (BYFLI), or 4) go immediately to streamflow via impaction on a variable saturated source area (SRFL). Water in the soil matrix (SWATI) moves vertically according to the Darcy Richards equation for saturated or unsaturated flow. A downslope flow component may also be simulated (DSFLI). Integration of these rates is by explicit forward difference (Euler), but with a variable iteration time step that limits changes in layer water content and in potential gradients. The relationships among matric potential, soil water content, and hydraulic conductivity are parameterized by a modified Clapp-Hornberger formulation with values given at field capacity. Water is added to groundwater by gravity drainage from the deepest soil layer. The groundwater component of streamflow (GWFL) is simulated as a fixed fraction of groundwater each day. A fixed fraction of the groundwater outflow may be deep seepage. Simulated streamflow is the sum of SRFL, BYFL, DSFL, and GWFL. This can be compared with measured streamflow if that is available.
BROOK2 is designed for forest land, has only a single soil layer, and has few parameters. BROOK90 is applicable to any land surface, can have from one to many soil layers, and has many physically-meaningful parameters.