Climate
Climate¶
Climate is the driving force for the hydrologic cycle. Spatial variation in solar radiation drives the weather resulting in evaporation, rainfall, and snow.
1. Precipitation¶
Precipitation is the measured rainfall. You can specify the precipitation as a rate, for example in [mm/hr], or as an amount, for example in [mm]. If you use the amount method, MIKE SHE will automatically convert this to a rate during the simulation.
If you use a rate, then the EUM Data Units must be “Precipitation” and the time series must be Mean Step Accumulated.
If you use an amount, then the EUM Data Units must be “Rainfall” and the time series must be Step Accumulated.
The Precipitation Rate item comprises both a distribution and a value. The distribution can be either uniform, sub area-based or fully distributed. If the data is sub area-based then for each station a sub-item will appear where you can enter the time series of values for the station.
2. Evapotranspiration¶
The calculation of evapotranspiration uses meteorological and vegetative data to predict the total evapotranspiration and net rainfall due to
- Interception of rainfall by the canopy,
- Drainage from the canopy to the soil surface,
- Evaporation from the canopy surface,
- Evaporation from the soil surface, and
- Uptake of water by plant roots and its transpiration, based on soil moisture in the unsaturated root zone.
The primary ET model is based on empirically derived equations that follow the work of Kristensen and Jensen (1975), which was carried out at the Royal Veterinary and Agricultural University (KVL) in Denmark. This model is used whenever the detailed Richards equation or Gravity flow methods are used in the Unsaturated zone.
In addition to the Kristensen and Jensen model, MIKE SHE also includes a simplified ET model that is used in the Two-Layer UZ/ET model. The Two-Layer UZ/ET model divides the unsaturated zone into a root zone, from which ET can occur and a zone below the root zone, where ET does not occur. The Two-Layer UZ/ET module is based on a formulation presented in Yan and Smith (1994). Its main purpose is to provide an estimate of the actual evapotranspiration and the amount of water that recharges the saturated zone. It is primarily suited for areas where the water table is shallow, such as in wetland areas.
The reference evapotranspiration (ET) is the rate of ET from a reference surface with an unlimited amount of water. Based on the FAO guidelines, the reference surface is a hypothetical grass surface with specific characteristics.
The reference ET value is independent of everything but climate and can be calculated from weather data. The FAO Penman-Monteith method is recommended for determining the reference ET value.
The reference ET is multiplied by the Crop Coefficient to get the Crop Reference ET. The Crop Coefficient is found in the Vegetation development table in the Vegetation database. If the vegetation database is not used, then the Reference ET is the maximum ET rate.
The Reference Evapotranspiration item comprises both a distribution and a value. The distribution can be either uniform, sub area-based or fully distributed. If the data is sub area-based then for each station a sub-item will appear where you can enter the time series of values for the station.
3. Freezing and Snow melt¶
MIKE SHE includes a comprehensive snow melt module based on a modified degree-day method. If the Include snow melt checkbox is checked then rain accumulates as snow if the Air Temperature is below the Threshold Melting Temperature (the temperature at which the snow starts to melt - usually 0 C). If the air temperature is above the threshold, then the snow will melt at the rate specified by the Degree-day Melting or Freezing Coefficient.
Dry snow acts like a sponge and does not immediately release melting snow. Thus, melting snow is added to wet snow storage. When the amount of wet snow exceeds the Maximum Wet Snow Fraction in Snow Storage, the excess is added to ponded water, which is then free to infiltrate or runoff.
More detailed information on the snow melt process can be found in the online help for the individual dialogues and in the Snow Melt - Technical Reference section.
Air Temperature¶
For snow melt, the air temperature is critical. However, the air temperature changes significantly with elevation. In areas with significant elevation changes, snow will accumulate in upland areas - often where there is limited weather data available. The elevation correction for air temperature allows you to specify an elevation for the temperature stations and a temperature change rate with elevation. During the pre-processing, a temperature change factor is calculated for each cell and the actual temperature in the cell is calculated during the simulation using this factor.
In terms of snow melt, the air temperature along with the degree day melting coefficient determine the amount of melting that can occur. If you have daily temperature data, it may be difficult to properly account for the diurnal melting and freezing cycles.
Air temperature can also be an important parameter during water quality simulations.
For more information on the snow melt parameters, see the specific snow melt dialogue information in the Climate section of the online help and User Interface manual.
Frozen Ground¶
Frozen ground is not directly simulated in MIKE SHE but can be approximated by using a time varying infiltration rate, where you manually reduce the infiltration rate during the winter.