IRRICALC
Irricalc IS Approved by Environment Canterbury to Determine Irrigator Seasonal Allocation Volumes
How to Calculate a Farms Seasonal Irrigation Water Limit Using the Irricalc Maps
This procedure is designed to provide seasonal irrigation water use limits that comply with Environment Canterbury’s policy that such limits be based on the assumption that irrigation application efficiency is 80% and provide sufficient water volume to meet reasonable irrigation water use 4 years out of 5, on average.
This procedure is also designed to provide seasonal irrigation water use limits for arable farms, as well as for pasture-based farms. The seasonal irrigation water use limit for an arable farm is based on the seasonal irrigation water use limit for the farm if the farm was pasture-based. The seasonal irrigation water use limit for the farm is determined for pasture first and then this number is converted to the arable farm equivalent.
Step 1: Determine which Soil Water Holding Capacity Class to Use for your Farm
Do this by locating where your farm is on the Soil Water Holding Capacity (Soil WHC) map for your area. Kaikoura, Culverdon Basin, North Canterbury, Central Canterbury, Mid Canterbury, South Canterbury, Lower Waitaki & MacKenzie Basin
Once you have located your farm on the Soil WHC map, use the colour at that location with the legend at the top left corner of the map to determine the Soil WHC for your farm.
If the soils information for a farm comes from a different source the following points need to be taken into account:
- Determining Soil WHC (mm) – The Soil WHC to be used with the Irricalc method is the soil water (mm) that is stored in the plant root zone between wilting point and field capacity. The depth of the plant root zone varies between crops/pasture/lucerne. For pasture a root zone depth of 600mm is recommended. Note that the Profile Available Water given in the ECan GIS system is for a soil depth of 900mm.
- Determining the Soil WHC Class – The Soil WHC class labelled as “60mm”, for example, represents the range of soil water holding capacities in the range 50mm to 69mm. The “80mm” class represents the range 70mm to 89mm, and so on.
Step 2: Find out the Seasonal Irrigation Water Limit for Pasture on your Farm
To do this you need to use the seasonal irrigation water use map that is labelled as the Soil WHC class of your farm AND 80% efficient irrigator AND 4 in 5 year reliability. Kaikoura, Culverdon Basin, North Canterbury, Central Canterbury, Mid Canterbury, South Canterbury, Lower Waitaki & MacKenzie Basin
Locate your farm on the map that is labelled with the Soil WHC class of your farm AND 80% efficient irrigator AND 4 in 5 year reliability.
Once you have located your farm on this map, use the colour at that location with the legend at the top left corner of the map to find the seasonal volume contours that the farm lies between.
Linearly interpolate between the contours that lie either side of the farm to get the seasonal irrigation water use limit for that farm.
Step 3: If the farm is Arable, Convert the Seasonal Volume for a Pasture Farm to the Arable Equivalent
Arable Farm Seasonal Volume = Pasture-Based Farm Seasonal Volume multiplied by 0.65
Step 4: Convert the Seasonal Volume, in mm, to Seasonal Volume in cubic metres
Seasonal Volume in cubic metres = Seasonal Volume (in mm) x 10 x the area irrigated (in hectares)
GuidancE Notes for more detailed applications
What if you have Several Soil Types or Several Irrigation Systems on your Farm?
Work out the seasonal volume for each relevant combination of Soil WHC class on your farm AND 80% efficient irrigator AND 4 in 5 year reliability and add the volumes up
What if an Irrigator Traverses more than One Soil Type on its Run – Which Soil do I Use?
Sometimes an irrigator will irrigate more than one soil type on its run. In this case the operation of the irrigator will probably be based on the soil type that has the smallest soil water holding capacity. It is this Soil WHC that should be used to calculate the seasonal irrigation volume for that irrigator run. The seasonal volume should be calculated for each irrigator run (or circle) and added up to give the total for the farm.
In this situation a judgement call must be made about which Soil WHC to use for each irrigation run. Justification of the decision about which Soil WHC was used is required.
What if the Farm is Located in an Inland Basin, such as the Mackenzie Basin?
The IrriCalc method uses a crop factor that has been calibrated for growing conditions on the Canterbury Plains. The length of the growing season in inland, high altitude, basins is significantly shorter than the growing season on the Plains. This will change some characteristics of the crop factor to be used. The IrriCalc method should, in this situation, be used with a crop factor that has been constructed using information contained in FAO Paper 56, “Crop Evapotranspiration: Guidelines for computing crop water requirements”, 1998. Justification of any subjective decisions made by the user in the construction of the crop factor is required. Please contact Aqualinc for more information.
