How  RHESSys works

The RHESSys model is a Geographical Information Science (GIS) – based hydro-ecological modeling framework, which simulates how water, carbon and nutrients fluctuate through the environment on a watershed scale. RHESSys is an important tool in understanding the effects of human impacts on landscape-level processes. RHESSys has been implemented in a number of projects to assess the impacts of land use and disturbances on stream hydrology and forest productivity. The RHESSys model is an adaptation of several pre-existing models, including MTN-Clim , BIOME – BGC, TOPMODEL, and DHSVM.

GIS software required to produce maps and run RHESSys may include GRASS (Geographic Resources Analysis Support System), Unix, ESRI Arcview 3.3 and ArcInfo.

Figure 1 presents an overview of the inputs, outputs and flow of control in the RHESSys modeling system.

Required Data Input information for RHESSys

As indicated in Figure 1, the various data inputs required to run RHESSys include:

Leaf area index (LAI) map–the structural property of the plant canopy, used to describe the interception of rainfall and its fate in terms of photosynthesis, evaporation and transpiration of water from the plant and ground surface

Vegetation map– various attributes of vegetation will effect rainfall interception and water use

Soil map – soil texture influences the amount of infiltration and runoff, for example, clays have a low infiltration capacity yet are able to retain water after a rain event, and sand has a high infiltration capacity yet is less capable of holding onto water after rainfall

Land use map – In RHESSys, the landscape is divided up into 4 different land uses – agricultural, urban, undeveloped, and residential. Each has varying capacities to allow water to infiltrate or run off. For example, urban areas are likely to have low infiltration and high runoff capabilities, whereas undeveloped areas are likely to have high infiltration  and low runoff capabilities.

Default files – For the land use, soil and vegetation type maps described above, default files can be can be used that describe their various parameters that affect water infiltration and runoff.

K  (hydraulic conductivity) map – K is directly related to soil type, it relates to a soils ability to transmit water. K is highest in sandy soils and lowest in clay  based soils

m (decay of saturated hydraulic conductivity with depth) map – m  relates directly to soil type and K. K will change as soil texture properties change with depth and m reflects these changes

An impervious surface map – this map is based on various land surface abilities to absorb water on a scale of 0 to 1. A totally imperious area, such as a road, will be given a value of 0 and totally pervious surface, such as a forest, will be given a value of 1.

Road network map – required to produce the impervious map above

DEM – Digital elevation model  - A DEM is required to delineate several maps needed to run a RHESSys simulation, including:

·        a basin boundary map of an area that you are interested in studying the fluxes of water, carbon or nutrients through.

·        a hillslope map, which define areas which drain to a single point or stream reach.

·   a patch map, which divides the basin up spatially. Within each patch, a water balance (or nitrogen/carbon content) is calculated, using rates of infiltration, storage and runoff according to the soil, vegetation and land cover types and other parameters within the patch.

·        a stream network map to determine where water will flow

·        a slope map to determine how fast water will flow down a slope and in what direction. The speed of water flow will determine how much water is able to infiltrate and how much runs off.

·       an aspect map to determine the orientation of surfaces within the basin. Aspect determines such things as vegetation growth and soil moisture content, which affect runoff and infiltration.

·       Climate data – climate is an important factor in determining how water moves through a basin. Necessary data include minimum and maximum temperature and precipitation. Irrigation data may also be included as water input.

·        Streamflow data – this data will be necessary for calibrating the model

To view examples of these maps click here to link to a representation of the Campo Creek Sub basin (situated within the Tecate River sub basin) on the ArcIMs map services page.

Creating a landscape representation

Once all of the data have been collected and put into a format RHESSys can read, a worldfile, flow table and tec file need to be created to run the model.

Creating a worldfile  - A worldfile is simply a text file that describes the data properties and allows them to be represented in the landscape. To create a worldfile, a template file must first be created which will contain where the data is located and a set of instructions to create the worldfile.  A customizable template file is available  from the RHESSys website.

Creating a flow table - A flow table also needs to be created to express the connectivity of the various data layers in terms of water drainage throughout the basin. The flow table is created by exporting the appropriate files (basin, hillslope, zone, stream, road, K, m) as ascii files and running the create_flowpaths command  in GRASS.

Creating a tec file - A temporal event control (tec) file needs to be created to execute the start and finish of output printing options for results of the simulation on an hourly, daily, monthly or yearly basis.

Running a simulation

RHESSys will read properties of the objects in the landscape from the worldfile created above. RHESSys will also require access to the default variable files (from the RHESSys manual) mentioned in the data inputs above, as well as the climate data.

RHESSys output

After the simulation has been run, RHESSys will have produced a large tabular ASCII file of variables, such as plant growth and the resulting evaporation, plant respiration, photosynthesis values, and how they have changed through time. For water resources information, streamflow data is produced which tells us how much water is leaving the basin after a given amount of rainfall specified in the climate data input. To improve the quality of the simulation in terms of streamflow, the model must be calibrated with observed streamflow data, which may be obtained from USGS gauging stations located within the basin of interest.

Application of RHESSys  in the Tecate River Sub basin

To help determine the baseline hydrological conditions within the Tecate River Sub basin, a RHESSys simulation was run. So that the model could be calibrated, observed streamflow data from the Campo Creek gauging station was obtained. Unfortunately, this station is not located at the stream outlet of the Tecate River Sub basin, but some 20 miles upstream. As a result, for the purpose of calibration, it was necessary to delineate a sub-basin from this gauging station, thus producing another smaller sub-basin within the Tecate River Sub basin, known as the Campo Creek sub-basin. The extent of this sub-basin, and the GIS data layers required to run this simulation, may be viewed on the ARCIMS web page by following this link. Viewing these data layers may assist the reader in gaining a better understanding of the processes involved in watershed runoff and of the RHESSys model.

Future application of the RHESSys model in the Campo Creek Sub basin

In addition to helping scientists better understand the processes of water flow within a sub-basin, watershed modeling using RHESSys may also assist watershed planners in land use decisions. To estimate the effect of urbanization or other types of landscape changes on streamflow, watershed planners can adjust the land use data layer to reflect these changes and run the model. Streamflow data results can then be compared with results from the model baseline conditions to observe any change in the volume of water leaving the basin. Running the model under different land use scenarios may assist planners in choosing a future land use development strategy that will minimize loss of water from the watershed.

Figure 1. Inputs and outputs of RHESSys

Source: http://typhoon.sdsu.edu/Research/Projects/RHESSYS/intro.html