Working with Evapotranspiration - User Guide¶
The calculation of evapotranspiration uses meteorological and vegetative data to predict the total actual evapotranspiration 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, and its replenishment from the saturated zone due to capillarity.
In MIKE SHE, the ET processes are split up and modelled in the following order:
- A proportion of the rainfall is intercepted by the vegetation canopy, from which part of the water evaporates.
- The remaining water reaches the soil surface, producing either surface water runoff or percolating to the unsaturated zone.
- Part of the infiltrating water is evaporated from the upper part of the root zone or transpired by the plant roots.
- The remainder of the infiltrating water recharges the groundwater in the saturated zone where it will be extracted directly if the roots reach the water table, or indirectly if capillarity draws groundwater upwards to replace water removed from the unsaturated zone by the roots.
The primary ET model is based on empirically derived equations that follow the work of Kristensen & Jensen (1975), which was carried out at the Royal Veterinary and Agricultural University (KVL) in Denmark.
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.
1. Some ET Definitions¶
Evaporation is the when liquid water is converted to vapour due to the addition of energy. Evaporation of water occurs from any free water surface, such
as a lake. Evaporation requires energy, which is mostly supplied by solar radiation, and to a lesser extent by air temperature. Evaporation will slowly decrease to zero as the humidity of the air above the water surface increases. Thus, wind is critical to carry moist air away, and replace it with drier air. In the context of soil evaporation, plants will shade the soil, reducing solar radiation and reducing evaporation.
Transpiration is the evaporation of liquid water from the plant tissues. Plants primarily lose water through pores in their leaves called stomata. Different plant species are able to regulate their stomata openings to greater or lesser degrees. Thus, different plants will transpire at different rates. Similar to evaporation, the overall rate is determined by solar radiation, humidity and the wind. However, transpiration also depends on the status of the crop, cultivation practices, etc.
Evapotranspiration (ET) is the combination of both evaporation and transpiration on vegetated surfaces. Both processes occur simultaneously and cannot be separated. In the case of crops sown in bare soil, after sowing ET is 100% evaporation but as the plants grow and shade the soil, the transpiration begins to dominate.
Reference ET (\(ET_0\)) is the maximum ET that can be extracted from a standard grass surface (reference crop) that is fully watered. The \(ET_0\) is a climate parameter and is dependent only on the climate variables. It is NOT dependent on the vegetation or crop. \(ET_0\) is defined by the Penman-Montieth method. The most widely used reference document for the Penman-Montieth method is the FAO-56 document by Allen et al (1998).
The units of ET are normally the same as precipitation, eg mm/day. Evapotranspiration is a significant loss of water. One hectare is 10,000 m2. So an ET rate of 1mm/d is equivalent to 10 m3 of water per hectare per day.
Table 22.1 Average \(ET_0\) for different climatic regions in mm/day (from FAO-56)
| Regions | Mean daily temperature | ||
|---|---|---|---|
| Cool (\~10C) | Moderate (20C) | Warm (>30C) | |
| Sub-/Tropics | |||
| humid/sub-humid | 2-3 | 3-5 | 5-7 |
| arid/semi-arid | 2-4 | 4-6 | 6-8 |
| Temperate | |||
| humid/sub-humid | 1-2 | 2-4 | 4-7 |
| arid/semi-arid | 1-3 | 4-7 | 6-9 |
Leaf Area Index (LAI)¶
The Leaf Area Index (LAI) is defined as the one-sided, area of leaves above a unit area of the ground surface. Generalised time varying functions of the LAI
for most crops and types of vegetation are available in the literature. In MIKE SHE, you must specify the temporal variation of the LAI for each vegetation type during the growing seasons to be simulated. Different climatic conditions from year to year may require a shift of the LAI curves in time but will generally not change the shape of the curve. Typically, the LAI varies from 0 to a about 7 for forests and up to about 10 for dense plantation forests.
Root Depth¶
The root depth is defined as the maximum depth of active roots in the root zone.
2. ET from Canopy Interception¶
Interception is defined as the process whereby precipitation is retained on the leaves, branches, and stems of vegetation. This intercepted water evaporates directly without adding to the moisture storage in the soil.
The interception process is modelled as an interception storage, which must be filled before stem flow to the ground surface takes place. The size of the interception storage capacity depends on the vegetation type and its stage of development, which is characterised by the leaf area index, LAI.
Note
The interception coefficient is a unit of length [mm] - not a rate. This means that the full amount is intercepted in every time step, if precipitation is available and the storage is not full. Thus, the total amount of intercepted water is time step dependent. For example, if you have a precipitation rate of 2 mm/hour over 12 hours, the total precipitation will be 24 mm. However, the total interception could range between 2 mm if the time step length is 12 hours to the full 24 mm, if the time step length is 1 hour, assuming that there is 2 mm of evapotranspiration per time step.
The amount of soil water, which can be intercepted by the vegetation canopy is determined by multiplying the interception capacity, \(C_{int}\), by the LAI. \(C_{int}\) depends on the surface characteristics of the vegetation type. The units of \(C_{int}\) are [L], but they should be interpreted as [L]/(area of leaves)/(ground area). A typical value is 0.05 mm.
The calculation of soil evaporation contains two components, the basic soil evaporation which occurs regardless of soil dryness at moisture contents in the range \(\theta_W - ½(\theta_W + \theta_F)\) and enhanced soil evaporation at moisture contents above \(½(\theta_W + \theta_F)\). The fraction of the potential evapotranspiration, which is always allocated to the basic soil evaporation, is determined by C2. In the two-layer soil model described by Kristensen & Jensen (1975), this value was found to be 0.15. For dynamic simulation using the unsaturated zone description in MIKE SHE, a value of 0.2 was, however, found to give better results (Miljøstyrelsen (1981); Jensen (1983)).
The transpiration from the vegetation is regulated by two parameters. C1 is the slope of the linear relation between LAI and Ea/Ep, which determines at
which LAI the actual evapotranspiration equals the potential evapotranspiration at ample water supply. A typical value of C1 is 0.3. C3 regulates the influence of water stress on the transpiration process and may depend on the soil type with higher values for light soils than for heavier soils. The influence of soil dryness is reduced when C3 is increased. In Kristensen & Jensen (1975), a value of 10 mm was found for loamy soils. For simulations with the unsaturated zone description in MIKE SHE, a value of 20 mm was found more appropriate (Miljøstyrelsen (1981); Jensen (1983)).
The root distribution in the soil is regulated by the Aroot parameter. The value of Aroot may depend on soil bulk density with higher values for soils with high bulk density where root development may be more restricted than for soils with low bulk density. A typical value is 1 at which 60% of the root mass is located in the upper 20 cm of the soil at a root depth of 1 m. Lower Aroot values decrease th