Land Use¶
1. Land Use¶
The land surface plays a very important role in hydrology. In principle, the land use section is used to define the properties of the land surface. The most important of these is the distribution of vegetation, which is used by MIKE SHE to calculate the spatial and temporal distribution of actual evapotranspiration.
However, the land surface comes into play in many ways and other sections of the data tree also include properties related to land use. Some of these properties are related to the vegetation distribution and may even be spatially identical. For example:
Topography¶
The topography is a physical property of the land surface that defines the hydraulics of both the overland flow and the unsaturated flow. See Topography. Related to topography is the definition of Sub-catchments, which is needed when you are using the Linear Reservoir method for groundwater or the simple, catchment based overland flow method.
Flood zones¶
In MIKE SHE, flood zones can be defined relative to the river model branches using Flood codes. For details on how use Flood codes see the chapter on Surface Water.
Hydraulic properties¶
The properties related directly to overland sheet flow are found under Overland Flow. This includes the Manning number or surface roughness and the Detention Storage, both of which are influenced or even defined by the vegetation.
Hydraulic flow¶
Areas of the land surface can be hydraulically divided by man-made structures, such as road ways and embankments, which can be defined by Separated Flow Areas.
Infiltration properties¶
The infiltration rate is a property of the soil type, which may be modified by the land use. Related to the gross infiltration rate is the presence or absence of macropores and other soil features leading to rapid infiltration. Both of these properties are found in the Unsaturated Flow section. However, land surface sealing and compaction can be defined as a reduced contact between ponded water and the subsurface. This is defined in the Overland flow section as a Surface-Subsurface Leakage Coefficient.
Groundwater drainage¶
As the groundwater table rises, it intersects low lying topographic features, such as ditches, or other man-made drainage features, such as buried farm drains. These features are related to land use, but are specified as Groundwater drainage
Overland drainage and Paving¶
Heavy rainfall will generate ponded areas in the landscape and paving will inhibit infiltration. However, 2D overland flow does not usually travel far. It typically drains into local, small-scale ditches and channels. The overland drainage function is used to route local ponded water and rainfall to urban and natural drainage features.
2. Vegetation¶
The vegetation properties are used to calculate the Actual evapotranspiration from the Reference evapotranspiration defined under Climate.
The primary vegetation properties are Leaf Area Index (LAI) and Root Depth (RD). The vegetation parameters can be specified in two different ways. They can either be specified directly as uniform values, time series, sub areas, gridded values or time varying grids, or, they can be defined as a crop rotation in the Vegetation Properties Editor.
A good source of local information on LAI and root depth is the agronomy department at your local university.
Leaf Area Index¶
The LAI is defined as the area of leaves per area of ground surface. The LAI values are characteristic of the plant type, season, and plant stress. LAI values are widely available in the literature for most major plant types.
The LAI is a lumped parameter for a cell that defines the average leaf area of the cell. In forests, it includes both the leaf area of the forest canopy and the understory. In more open areas, it is an average for all vegetation types, such as grass, brush and trees. In areas of largely open water the LAI is usually zero. If the LAI is zero, there will be no interception storage, and no water will be removed from the unsaturated zone.
Root Depth¶
Root depth is defined as the depth below ground in millimeters to which roots extend. The root depth is not necessarily the average root depth. In some cases, it may be the maximum root depth. The root depth defines the depth at which water can be extracted from the unsaturated zone. If the root depth is deeper than the depth of the capillary zone, then the roots will be able to extract water from the saturated zone.
The thickness of the capillary zone is defined by the soils function in the soil properties for the Richards and Gravity flow methods. In the 2Layer UZ method, the thickness of the capillary zone is defined by the ET Surface Depth.
If you are using the Richards or Gravity Flow UZ methods, then you will also be able to use the Root Shape factor (AROOT) for each vegetation type. This allows you, for example, to extract more water from the upper UZ cells than the lower cells, which is typical of grasses in semi-arid climate zones.