IrrigationSchedule

Syntax

IrrigationSchedule(CropNumber1, IrrigationStartDate1, IrrigationEndDate1, IrrigationTriggerMethod1, IrrigationTriggerValue1, IrrigationAmountMethod1, IrrigationAmountValue1, CropNumber2, IrrigationStartDate2, IrrigationEndDate2, IrrigationTriggerMethod2, ...)

Description

This function is used in conjunction with the MABIA Method for catchment hydrology and crop water requirements.

Irrigation is required when rainfall is insufficient to compensate for the water lost by evapotranspiration. The primary objective of irrigation is to apply water at the right period and in the right amount. By calculating the soil water balance of the root zone on a daily basis, the timing and the depth of future irrigation can be planned. To avoid crop water stress, irrigation should be applied before or at the moment when the readily available soil water (RAW) is depleted (Depletion >= RAW).

The IrrigationSchedule function specifies the timing and amount of irrigation. Each crop can have many different schedules for non-overlapping periods of the crop season. For example, you may want to irrigate more frequently during the sensitive flowering and yield formation stages. If a branch has more than one crop in rotation, you can specify the irrigation schedules for all crops. Use the Irrigation Scheduling Wizard to help you fill in the parameters for the IrrigationSchedule function.

CropNumber_i: To which crop does schedule i apply. 1 is the first crop in the rotation, 2 is the second crop (if any), and so forth. The same crop can have multiple schedules

IrrigationStartDate_i: The date on which schedule i begins, in Month Day format, e.g., "Jan 1" for January 1st.

IrrigationEndDate_i: The date on which schedule i ends.

IrrigationTriggerMethod_i: The method for determining when irrigation will occur. There are four "trigger" methods:

Fixed Interval: Irrigate every N days, where N is specified in the IrrigationTriggerValue.
% of RAW: Irrigate when soil moisture depletion is greater than or equal to a specified % of Readily Available Water (RAW). To prevent crop water stress, depletion should never exceed RAW.
% of TAW: Irrigate when soil moisture depletion is greater than or equal to a specified % of Total Available Water (TAW). To prevent crop death (permanent wilt point), depletion should never equal or exceed TAW.
Fixed Depletion: Irrigate when soil moisture depletion is equal to or exceeds a specified depth (in mm).

IrrigationTriggerValue_i: The value that goes with the IrrigationTriggerMethod: days (fixed interval), % (% of RAW or % of TAW), mm (fixed depletion).

IrrigationAmountMethod_i: The method for determining how much water to apply on days when irrigation occurs. There are four methods:

% Depletion: Apply a specified % of the current soil water depletion.
% of RAW: Apply a specified % of the Readily Available Water (RAW) level, regardless of the current soil water depletion.
% of TAW: Apply a specified % of the Total Available Water (TAW) level, regardless of the current soil water depletion.
Fixed Depth: Apply a specified depth of water. Note: the value entered should be the average depth of water applied to the entire area. If the fraction wetted (Fw) < 1, then the amount actually delivered to the crop will be higher: I/Fw . For example, for a drip irrigation system with Fw=0.3, a fixed depth application of 10 mm of water would be mean that 10/0.3 = 33.3 mm of water was applied to that 30% of the land area, and would be available for ET by the crop.

IrrigationAmountValue_i: The value that goes with the IrrigationAmountMethod: % (% Depletion, % of RAW or % of TAW), mm (fixed depth).

The optimal irrigation schedule and amount would use % of RAW (100% of RAW) as the trigger method, and % of Depletion (100% of Depletion) as the irrigation amount method, which would apply irrigation at the last moment before crop stress would occur, and irrigate just enough to get back up to field capacity. However, in reality, it will be difficult for a farmer to know exactly when depletion reaches the RAW threshold

For the first three irrigation amount methods (% of Depletion, RAW or TAW), the amount calculated will be the actual amount delivered to the crop. If the fraction wetted (Fw) < 1, then the average amount over the entire area will be less: I * Fw . For example, a drip irrigation system with Fw=0.3, a % of depletion amount method (100%), and a depletion of 40 mm, would require an average depth of irrigation of 40 * 0.3 = 12 mm. For a 1 hectare plot of land, this would be 1 ha * 12 mm = 120 m^3 of water. When the 120 m^3 of water was applied to the 30% of the 1 hectare, the crop would receive 120 m^3 / (30% * 1 ha) = 40mm.

If there are gaps in the irrigation dates between schedules, there will be no irrigation during those periods. There is no irrigation in the fallow periods (before or after the crop season).

Note: the irrigation amount is the amount available for evapotranspiration. If the irrigation efficiency is less than 100%, then the supply requirement for irrigation will be increased. For example, if the irrigation efficiency is 75% (which means that 25% is lost to evaporation, runoff or deep percolation) and the irrigation amount is 100% of depletion, and the depletion is 40mm, then the amount applied will be 40 / 0.75 = 53.3 mm, of which 40 mm will effectively reach the crop and be available for ET, and 13.3 will evaporate, runoff or percolate (as specified by Loss to Groundwater and Loss to Runoff fractions entered as data).

Examples

IrrigationSchedule(1, Mar 1, Nov 25, % of RAW, 100, % Depletion, 100)

This example has 1 irrigation schedule, triggering irrigation when soil moisture depletion reaches 100% of RAW, and will apply enough water erase 100% of the current soil moisture depletion.

IrrigationSchedule(1, Mar 1, Mar 30, Fixed Interval, 3, Fixed Depth, 80, 1, Mar 31, Sep 11, % of RAW, 100, % Depletion, 100, 2, Sep 12, Jan 29, % of RAW, 100, % Depletion, 90)

This example has 3 irrigation schedules--2 for the first crop and 1 for the second crop. The first crop will be irrigated every 3 days to a depth of 80 mm from March 1 to March 30, and from March 31 to September 11 will irrigation when depletion reaches 100% of RAW and apply an amount to eliminate all the soil moisture depletion (i.e., up to field capacity). The second crop will be irrigated from September 12 to January 29 when depletion reaches 100% of RAW and apply an amount equal to 90% of the soil moisture depletion (not quite enough to get back to field capacity).