Any budget involves comparing inputs and outputs, and making sure you expect to stay within some limits. You might like to think of soil water budgeting for irrigation like keeping tabs on your vehicle’s fuel use and reserves. (Soil moisture measurement shows the actual amount in the tank!)

If you are planning to drive somewhere, you’ll look at what is in the tank, think about how much fuel you expect to use per 100 km, and estimate if you have enough. If you don’t, you plan to put some more in. But you can’t put in more than the tank holds, or you get a lot of runoff! You might need 120 L to drive to a distant city, but if your tank only holds 60 L you’ll have to fill up at least twice.

In the same way, if you expect to need 70 mm of irrigation to last two weeks, but your soil only holds 40 mm, you’ll have to apply it in a couple of irrigations.

Some Definitions

Below we have defined a few terms that we will use in compiling a water budget. There is a lot more about soil water terminology available on the Food and Agriculture Organisation (FAO) website.

Evapotranspiration (ET)

ET is the water loss from leaf transpiration and soil evaporation – it is measured like rain in mm. ET is like negative rain

Potential Evapotranspiration (PET)

PET is the amount of water that would be lost by well-watered, well fed, healthy grass like a lawn at a research station

Crop Factor (F_crop)

The crop factor accounts for different species transpiring at different rates AND for the amount of ground covered by the crop. In general, bare soil loses water at roughly half the rate the watered plants do. Crop factors based on the effective ground cover are given for a range of vegetable crops grown in New Zealand in Table 1 below, prepared by Dr Tony Davoren. Notice that different crops (potatoes, lucerne) can have different crop factors for the same ground cover percentage.

The crop factors are based on thousands of actual soil moisture measurements made over many years in crops across New Zealand. They give a good guide for estimating likely water use, but you should still measure the soil moisture to keep a check on what is actually “in the tank”.

Crop Evapotranspiration (ET_crop)

The ET_crop is calculated by multiplying the PET by the relevant crop factor – which for most crops changes during the season. Let‘s look at irrigation for a bean crop as an example.

Let’s say your weather station or published information tells you the potential evapotranspiration tomorrow will be 4 mm. But you are not growing grass, you are growing a crop of beans that was planted about two months ago. You do a field check and find the plants cover 40% of the ground area, so you adjust the PET figure accordingly.

In this case, 4 mm X 0.40 (or 40%) means you expect to use about 1.6 mm of water tomorrow.

Effective Rainfall

Effective rainfall only includes that which can be stored in the crop’s root zone. If there is a lot of rain or irrigation (or both) much of the water that lands will be lost through drainage or runoff.

Readily Available Water (RAW)

In this context, Readily Available Water is the amount of water held in the root zone between Field Capacity and Stress Point. It is measured in millimetres rainfall equivalent.

NOTE: As the crop’s roots grow deeper, the amount of soil it can get water from increases.  Budgets change as crops develop, and as the weather changes.

Tracking Deposits and Withdrawals

The FAO diagram below shows the main ins and outs for a soil water budget.

The inputs we can measure are irrigation and rainfall – both in millimetres. We can estimate outputs from PET figures and crop adjustments.

We do the maths and try to keep the soil moisture between field capacity and stress point (or refill point as it is also called). We calculate wasted water via run off and deep percolation in our budget –  as this is how we account for the excess we applied that put the soil moisture over field capacity – the litres of fuel we spilt across the forecourt.

We can’t do much about many of the other factors such as capillary rise and subsurface flow, so we usually ignore them, and most times we’ll be alright as if we have flowing springs and upwelling water tables maybe we don’t need to irrigate then anyway.

Bringing it together

An example of a water budget is shown in the table below. It tracks inputs (rain and irrigation) and outputs (ET_crop and drainage) and tallies the water balance (Column M). You can see that the crop factor slowly increases as the crop grows and covers ore of the ground.

The balance in the soil can never exceed the Readily Available Water of 40 mm. If there is too much water added, it gets lost as drainage. So after September rain, we see the soil draining the excess water.

Here is a simple chart tracking the inputs, drainage losses and the soil water balance. You can see the rainfall data as pale blue bars, and irrigation as dark blue bars. The excess drainage is shown as red bars. The green line tracks the soil water balance through September, and then on until mid-February. Budgets and charts like these let you try out different irrigation strategies.

Watch a video describing soil water budgeting here: