11 Irrigation
The SWAP agrohydrological simulations are useful to develop optimal irrigation schedules by evaluating various application strategies. Irrigation strategies may be applied with a fixed or a scheduled regime. The fixed regime is defined by the time and depth of irrigation application (Section 11.1). The scheduled regime is defined by different criteria for time and depth of an irrigation application (Section 11.2). A combination of a fixed and a scheduled regime is also possible. This regime allows the evaluation of water productivity in relation to several degrees of water stress. Irrigation or flooding may also be simulated using the supply of water from a nearby field or from surface waters (Section 11.3). This chapter ends with a description of input requirements (Section 11.4). Three types of irrigation can be considered: overhead sprinkling irrigation, surface irrigation and subsurface irrigation. For sprinkling irrigation SWAP calculates interception in the same way as for rainfall interception.
11.1 Fixed irrigation regime
At user-defined dates, a fixed application depth (amount) may be applied as an observed gross irrigation dose (\(I_g\)). Interception of irrigation water may occur, dependent on the type of application (surface irrigation or sprinkling):
\[ I_n = I_g - E_i \tag{11.1}\]
where \(I_n\) is the net amount of irrigation water (cm d-1), \(I_g\) is the gross given amount of fixed irrigation water (cm d-1), \(E_i\) is the amount of intercepted irrigation water (cm d-1). The interception irrigation water is assumed to evaporate on the same day as the irrigation occurs. When irrigation supply should not be evenly spread over the day, in addition the irrigation supply rate (mm h-1) must be provided.
11.2 Scheduled irrigation regime
A specific combination of timing and depth criteria is valid from a user-defined date in the growing season until the end of crop growth. Both timing and depth criteria may be defined as a function of crop development stage. Scheduled irrigation only occurs when a crop is present.
11.2.1 Timing criteria
For the timing of the irrigation schedule, one out of five different criteria must be selected:
- Allowable daily stress
- Allowable depletion of readily available water in the root zone
- Allowable depletion of totally available water in the root zone
- Allowable depletion amount of water in the root zone
- Critical pressure head or moisture content at sensor depth
At the start of each day, the selected criterion is evaluated based on state variables at the start of the day. The outcome of this evaluation may generate an irrigation event on that same day. The five different evaluation criteria are explained in Section 11.2.1.1 to Section 11.2.1.5. In addition, the user may prescribe a fixed or a minimum interval between irrigated applications (Section 11.2.1.6 and Section 11.2.1.6).
11.2.1.1 Allowable daily stress
The level of soil water shortage by drought and salinity stress may be diagnosed from a threshold defined by the ratio of reduced transpiration \(T_r\) to potential transpiration \(T_p\) that occurred during the previous day. Irrigation is applied whenever reduced transpiration becomes lower than a limit defined by this threshold:
\[ T_r \leq f_1 T_p \tag{11.2}\]
where \(T_r\) is the transpiration reduced by drought and salinity stress (cm d-1), \(T_p\) is the potential transpiration (cm d-1), \(f_1\) (-) is a user-defined factor [0..1] for allowable daily stress.
11.2.1.2 Allowable depletion of readily available water
In order to optimize irrigation scheduling where irrigation is always secured before conditions of soil moisture stress occur, the maximum amount of depletion of readily available water in the root zone can be specified. Irrigation is then applied whenever the water depletion exceeds fraction \(f_2\) of the readily available water amount:
\[ (U_{field} - U_a) \geq f_2 (U_{field} - U_{h3}) \tag{11.3}\]
where \(U_a\) (cm) is the actual water storage in the root zone, \(U_{field}\) (cm) is the root zone water storage at \(h = h_{FC}\) given value for field capacity, and \(U_{h3}\) (cm) is the root zone water storage at \(h = h_3\), the pressure head from where root water extraction starts being reduced due to drought stress, \(f_2\) (-) is a user-defined depletion fraction.
\(U_a\) is calculated by integrating numerically the water content in the rooting layer. For deficit irrigation purposes, stress can be allowed by specifying \(f_2 > 1\).
11.2.1.3 Allowable depletion of totally available water
Depletion of water in the root zone can also be evaluated relative to the total amount of water available in the root zone as given by the difference between the field capacity and the wilting point. Irrigation is then applied whenever the depletion of water in the root zone exceeds fraction \(f_3\) (-) of the available water:
\[ (U_{field} - U_a) \geq f_3 (U_{field} - U_{h4}) \tag{11.4}\]
where \(U_{h4}\) (cm) is the root zone water storage at \(h = h_4\), the pressure head at which root water extraction is reduced to zero, \(f_3\) (-) is a user-defined factor depletion fraction.
11.2.1.4 Allowable depletion of field capacity water
In case of high-frequency irrigation systems (drip), it may be useful to specify the maximum amount of water that may be extracted below field capacity, \(\Delta U_{max}\) (cm). Irrigation is then applied if:
\[ U_a \leq U_{field} - \Delta U_{max} \tag{11.5}\]
11.2.1.5 Critical pressure head or moisture content
The user may specify a soil moisture threshold value \(\theta_{min}\) (cm3 cm-3) or pressure head threshold value \(h_{min}\) (cm) and a corresponding depth for which the threshold values are valid. This option may be used to simulate irrigation with automated systems relying on soil moisture measurements. Irrigation is then applied whenever a threshold is exceeded:
\[ \theta_{sensor} \leq \theta_{min} \quad \text{or} \quad h_{sensor} \leq h_{min} \tag{11.6}\]
where \(\theta_{sensor}\) and \(h_{sensor}\) are the sensor values for soil moisture and pressure head, which are simulated by SWAP. In addition, the user may specify a salinity threshold value above which over-irrigation is applied to decrease the high salinity concentrations.
11.2.1.6 Fixed interval
By default, an irrigation interval has a minimum of one day and the length of the interval is variable and determined by the moment when one of the previously mentioned timing criteria becomes valid. The user may optionally choose a fixed interval of one week between possible irrigation events. Irrigation events occur weekly during crop growth when the required amount of water to bring the root zone to field capacity exceeds a given threshold value. This threshold value is input to the model.
11.2.1.7 Minimum interval
The length of the interval between irrigation events may also be variable and be determined by the moment when one of the timing criteria becomes valid. The user may select this option in addition to one of the previous five criteria (Section 11.2.1.1 to Section 11.2.1.5) to have a minimum time interval between irrigation applications.
11.2.2 Depth criteria
Scheduled irrigation results in gross irrigation depths (amounts). Interception of irrigation water may occur in case of sprinkling irrigation:
\[ I_n = I_g - E_i \tag{11.7}\]
Two options are available for the amount of irrigation:
- An amount which increases root zone soil moisture to field capacity
- A fixed amount
The actual amount of the application may be limited to a minimum and a maximum amount. Both criteria are explained in more detail hereafter.
11.2.2.1 Back to field capacity
The soil water content in the root zone is brought back to field capacity. This option may be useful in case of sprinkler and micro irrigation systems, which allow variation of irrigation application depth. An additional irrigation amount (\(dI\)) can be defined to leach salts, while the user may define a smaller irrigation amount when rainfall is expected. This amount is added to field capacity when the rain amount is larger than a specified threshold value (see Tip 11.2).
11.2.2.2 Fixed irrigation depth
A specified amount of water is applied. This option applies to most gravity systems, which allow little variation in irrigation application depth.
11.2.2.3 Limited depth
With this option enabled, the calculated irrigation depth \(I_g\) will be limited to the range:
\[ I_{g,min} \leq I_g \leq I_{g,max} \tag{11.8}\]
where \(I_{g,min}\) and \(I_{g,max}\) are the specified minimum and maximum irrigation amount values (mm), respectively.
11.2.3 Irrigation supply rate
By default, irrigation is supplied evenly spread over the day. Actual supply rates may be considered by providing the irrigation supply rate (mm h-1). In that case the end-time of the irrigation event will be calculated as \[ \Delta t_{irr} = \frac{I_g}{24I_r} \tag{11.9}\] where \(\Delta t_{irr}\) is the finish time of the irrigation event (fraction of current day; d), \(I_g\) is the irrigation depth (mm), \(I_r\) is the irrigation supply rate (mm h-1) and 24 is a time conversion factor (h d-1). As \(\Delta t_{irr}\) may not exceed 1, it is maximized by 1 by temporarily adapting \(I_r\) (see warning file). The value for \(\Delta t_{irr}\) affects the time controller in SWAP.
11.2.4 Subsurface irrigation depth
When subsurface irrigation is considered the water supply occurs at a given depth: \(Z_{irr}\). For the computational soil layer where \(Z_{irr}\) is located a water source term equal to the supply rate \(I_r\) is added to the Richards equation (Equation 2.3). Alternatively, a depth range may be considered in which case the total supply is evenly distributed over the computational layers inside this depth range.
11.3 Flooding
Both fixed and scheduled irrigation may result in flooding conditions. Flooding may also be generated by increasing surface water levels or by adding water from a nearby field (runon). The option for input of surface water levels is described in Chapter 5. The option for runon for nearby fields as input to the model is described in Section 4.5.1.
11.4 User instructions
11.4.1 Fixed irrigation regime
Fixed irrigation depths must be entered in the *.swp file (Tip 11.1).
11.4.2 Scheduled irrigation
Scheduled irrigation enters the model by means of general, timing, and depth criteria in the *.crp file, see respectively Tip 11.2.