## 2.26.3. The irrigation model[¶](#the-irrigation-model "Permalink to this headline")
-----------------------------------------------------------------------------------

The CLM includes the option to irrigate cropland areas that are equipped for irrigation. The application of irrigation responds dynamically to the soil moisture conditions simulated by the CLM. This irrigation algorithm is based loosely on the implementation of [Ozdogan et al. (2010)](https://escomp.github.io/ctsm-docs/versions/master/html/tech_note/References/CLM50_Tech_Note_References.html#ozdoganetal2010).

When irrigation is enabled, the crop areas of each grid cell are divided into irrigated and rainfed fractions according to a dataset of areas equipped for irrigation ([Portmann et al. 2010](https://escomp.github.io/ctsm-docs/versions/master/html/tech_note/References/CLM50_Tech_Note_References.html#portmannetal2010)). Irrigated and rainfed crops are placed on separate soil columns, so that irrigation is only applied to the soil beneath irrigated crops.

In irrigated croplands, a check is made once per day to determine whether irrigation is required on that day. This check is made in the first time step after 6 AM local time. Irrigation is required if crop leaf area \\(>\\) 0, and the available soil water is below a specified threshold.

The soil moisture deficit \\(D\_{irrig}\\) is

(2.26.19)[¶](#equation-25-61 "Permalink to this equation")\\\[\\begin{split}D\_{irrig} = \\left\\{ \\begin{array}{lr} w\_{target} - w\_{avail} &\\qquad w\_{thresh} > w\_{avail} \\\\ 0 &\\qquad w\_{thresh} \\le w\_{avail} \\end{array} \\right\\}\\end{split}\\\]

where \\(w\_{target}\\) is the irrigation target soil moisture (mm)

(2.26.20)[¶](#equation-25-62 "Permalink to this equation")\\\[w\_{target} = \\sum\_{j=1}^{N\_{irr}} \\theta\_{target} \\Delta z\_{j} \\ .\\\]

The irrigation moisture threshold (mm) is

(2.26.21)[¶](#equation-25-63 "Permalink to this equation")\\\[w\_{thresh} = f\_{thresh} \\left(w\_{target} - w\_{wilt}\\right) + w\_{wilt}\\\]

where \\(w\_{wilt}\\) is the wilting point soil moisture (mm)

(2.26.22)[¶](#equation-25-64 "Permalink to this equation")\\\[w\_{wilt} = \\sum\_{j=1}^{N\_{irr}} \\theta\_{wilt} \\Delta z\_{j} \\ ,\\\]

and \\(f\_{thresh}\\) is a tuning parameter. The available moisture in the soil (mm) is

(2.26.23)[¶](#equation-25-65 "Permalink to this equation")\\\[w\_{avail} = \\sum\_{j=1}^{N\_{irr}} \\theta\_{j} \\Delta z\_{j} \\ ,\\\]

Note that \\(w\_{target}\\) is truly supposed to give the target soil moisture value that we’re shooting for whenever irrigation happens; then the soil moisture deficit \\(D\_{irrig}\\) gives the difference between this target value and the current soil moisture. The irrigation moisture threshold \\(w\_{thresh}\\), on the other hand, gives a threshold at which we decide to do any irrigation at all. The way this is written allows for the possibility that one may not want to irrigate every time there becomes even a tiny soil moisture deficit. Instead, one may want to wait until the deficit is larger before initiating irrigation; at that point, one doesn’t want to just irrigate up to the “threshold” but instead up to the higher “target”. The target should always be greater than or equal to the threshold.

\\(N\_{irr}\\) is the index of the soil layer corresponding to a specified depth \\(z\_{irrig}\\) ([Table 2.26.4](#table-irrigation-parameters)) and \\(\\Delta z\_{j}\\) is the thickness of the soil layer in layer \\(j\\) (section [2.2.2](https://escomp.github.io/ctsm-docs/versions/master/html/tech_note/Ecosystem/CLM50_Tech_Note_Ecosystem.html#vertical-discretization)). \\(\\theta\_{j}\\) is the volumetric soil moisture in layer \\(j\\) (section [2.7.3](https://escomp.github.io/ctsm-docs/versions/master/html/tech_note/Hydrology/CLM50_Tech_Note_Hydrology.html#soil-water)). \\(\\theta\_{target}\\) and \\(\\theta\_{wilt}\\) are the target and wilting point volumetric soil moisture values, respectively, and are determined by inverting [(2.7.53)](https://escomp.github.io/ctsm-docs/versions/master/html/tech_note/Hydrology/CLM50_Tech_Note_Hydrology.html#equation-7-94) using soil matric potential parameters \\(\\Psi\_{target}\\) and \\(\\Psi\_{wilt}\\) ([Table 2.26.4](#table-irrigation-parameters)). After the soil moisture deficit \\(D\_{irrig}\\) is calculated, irrigation in an amount equal to \\(\\frac{D\_{irrig}}{T\_{irrig}}\\) (mm/s) is applied uniformly over the irrigation period \\(T\_{irrig}\\) (s). Irrigation water is applied directly to the ground surface, bypassing canopy interception (i.e., added to \\({q}\_{grnd,liq}\\): section [2.7.1](https://escomp.github.io/ctsm-docs/versions/master/html/tech_note/Hydrology/CLM50_Tech_Note_Hydrology.html#canopy-water)).

To conserve mass, irrigation is removed from river water storage (Chapter [2.14](https://escomp.github.io/ctsm-docs/versions/master/html/tech_note/MOSART/CLM50_Tech_Note_MOSART.html#rst-river-transport-model-rtm)). When river water storage is inadequate to meet irrigation demand, there are two options: 1) the additional water can be removed from the ocean model, or 2) the irrigation demand can be reduced such that river water storage is maintained above a specified threshold.

Table 2.26.4 Irrigation parameters[¶](#id23 "Permalink to this table")  
| Parameter
 |  |
| --- | --- |
| \\(f\_{thresh}\\)

 | 1.0

 |
| \\(z\_{irrig}\\) (m)

 | 0.6

 |
| \\(\\Psi\_{target}\\) (mm)

 | \-3400

 |
| \\(\\Psi\_{wilt}\\) (mm)

 | \-150000

 |
