Net Depth vs Gross Depth: How Much Water to Apply Each Time

Net depth vs gross depth is the difference between the water plants can use and the water you must apply to deliver that amount to the root zone. In the worked example, loam with 30 cm roots holds 51 mm, net depth is 25.5 mm at 50 percent allowable depletion and gross depth is 31.9 mm at 80 percent efficiency.

Net depth answers how much water the soil can give up before plants start to struggle. Gross depth answers how much water to put on the soil to replace that amount after losses.

Net Depth vs Gross Depth in Plain Terms

Net depth is the depth of water stored in the root zone that plants can use before the next watering. It is a soil number, not a tap number. It depends on how much water the soil holds, how deep the roots go, and how far you let the soil dry. Gross depth is the depth you apply. It starts with net depth and adds enough extra to cover the water that never reaches the root zone.

Losses happen in every system. Some water evaporates before it soaks in, some runs off the surface, and some drains past the roots. The calculator page calls the share that reaches the root zone the efficiency, and it uses 80 percent in its worked example.

How Net Depth Is Worked Out

Net depth comes from two steps. First, root zone water equals available water percent times root depth. Second, net depth equals root zone water times allowable depletion, the share plants can use before stress affects growth.

Available water is the water a soil holds between field capacity and wilting point. The calculator page gives three verified values: sand about 7 percent, loam 17 percent, and silt loam 20 percent.

Root depth matters just as much. A shallow root system holds a small store, so it needs water often. A deep root system holds a larger store, so it can wait longer. Young plants need shorter intervals. The calculator page keeps 50 percent allowable depletion as the default, and the usual range is 30 to 60 percent.

How Gross Depth Is Worked Out

Gross depth comes from one step after net depth. The formula is gross depth equals net depth divided by efficiency. Efficiency is written as a decimal, so 80 percent becomes 0.80. Dividing by a number below one gives a larger answer, because gross depth must be larger than net depth whenever water is lost.

Take the worked example numbers. Net depth is 25.5 mm and efficiency is 80 percent. Gross depth is 25.5 divided by 0.80, which is 31.875 mm, shown as 31.9 mm. The gap is 6.4 mm, and it covers losses, so the amount left in the root zone is back to 25.5 mm. Gross depth does not change the interval. The interval comes from net depth divided by daily ET.

The Worked Example Step by Step

The worked example on the Irrigation Scheduling Calculator uses loam with 30 cm roots, 50 percent depletion, daily ET of 5 mm a day, and 80 percent efficiency.

1. Root zone water: 0.17 times 300 mm gives 51 mm.
2. Net depth: 51 mm times 0.50 gives 25.5 mm.
3. Interval: 25.5 mm divided by 5 mm a day gives 5.1 days, shown as about 5 days.
4. Gross depth: 25.5 mm divided by 0.80 gives 31.875 mm, shown as 31.9 mm.

Soil and roots set the store, depletion sets the usable share, daily water use sets the wait, and efficiency sets the amount.

Gross Depth at Example Efficiencies

The only efficiency given on the calculator page is 80 percent, from the worked example. The table below keeps net depth fixed at 25.5 mm. Every efficiency other than 80 percent here is an example efficiency only, not a page figure.

Example efficiency	Net depth	Gross depth
60 percent	25.5 mm	42.5 mm
70 percent	25.5 mm	36.4 mm
80 percent	25.5 mm	31.9 mm
90 percent	25.5 mm	28.3 mm
95 percent	25.5 mm	26.8 mm

Each row uses the same formula, gross depth equals net depth divided by efficiency. A drop from 80 percent to 60 percent lifts gross depth from 31.9 mm to 42.5 mm, an extra 10.6 mm at every watering. Do not read this table as a guide to your own system efficiency.

Net Depth and Gross Depth by Soil and Root Depth

Net depth changes with soil and root depth, and gross depth follows it. The tables below use only the three verified available water values, sand about 7 percent, loam 17 percent and silt loam 20 percent, with 50 percent allowable depletion. Root depths other than 30 cm are example inputs.

Soil	Root depth	Root zone water	Net depth at 50 percent depletion
Sand	20 cm	14.0 mm	7.0 mm
Sand	30 cm	21.0 mm	10.5 mm
Loam	30 cm	51.0 mm	25.5 mm
Loam	45 cm	76.5 mm	38.3 mm
Silt loam	30 cm	60.0 mm	30.0 mm
Silt loam	45 cm	90.0 mm	45.0 mm

Now apply the worked example efficiency of 80 percent to each net depth to get gross depth.

Soil and roots	Net depth	Gross depth at 80 percent efficiency
Sand, 20 cm roots	7.0 mm	8.8 mm
Sand, 30 cm roots	10.5 mm	13.1 mm
Loam, 30 cm roots	25.5 mm	31.9 mm
Loam, 45 cm roots	38.3 mm	47.8 mm
Silt loam, 30 cm roots	30.0 mm	37.5 mm
Silt loam, 45 cm roots	45.0 mm	56.3 mm

Each gross depth is net depth divided by 0.80. The loam 45 cm gross depth of 47.8 mm uses the unrounded net depth of 38.25 mm. Sand gives a small net depth even with deeper roots, so a sandy bed needs small depths often.

How Allowable Depletion Changes Both Depths

Allowable depletion changes net depth first, and gross depth follows. The table below keeps the soil fixed as loam with 30 cm roots, so root zone water stays at 51 mm, and it varies depletion across the usual range of 30 to 60 percent. Gross depth uses 80 percent efficiency.

Allowable depletion	Net depth	Gross depth at 80 percent efficiency
30 percent	15.3 mm	19.1 mm
40 percent	20.4 mm	25.5 mm
50 percent	25.5 mm	31.9 mm
60 percent	30.6 mm	38.3 mm

A lower depletion value gives a smaller depth and a shorter wait, which suits a sensitive crop or a young plant. A higher value gives a larger depth and a longer wait. Keep the default of 50 percent unless you have a clear reason to move it.

Why Applying Only the Net Depth Leaves Plants Short

This is the most common error with these two numbers. Net depth looks like the answer, because it is the number the plants need. The problem is that the amount you apply is not the amount that stays in the root zone.

Apply only the net depth of 25.5 mm with an 80 percent efficient system, and the stored amount is 25.5 times 0.80, which is 20.4 mm, or 5.1 mm short. Apply the gross depth of 31.9 mm instead, and the stored amount is 31.9 times 0.80, which is 25.52 mm, which rounds to 25.5 mm. Use net depth to plan the interval, and gross depth to set the amount.

How to Check the Depth You Really Applied

Check the applied depth with a straight sided container, such as a rain gauge, set on the soil surface. Run the system for the time you planned, then read the depth in millimetres and compare it with the gross depth target.

If the gauge reads 24.0 mm when the target is 31.9 mm, you are 7.9 mm short, and you need a longer run. If the gauge reads 40.0 mm, you are 8.1 mm over, and you need a shorter run. Place more than one gauge if the area is large. Different readings show uneven coverage, so set the run time from the lowest reading.

Rain changes the plan too. The calculator page says to subtract effective rainfall from the next irrigation, so measure rain in the same gauge and water only the remainder. The Garden Watering Schedule Calculator works the same way with weekly rain. If you need a daily ET figure, the Crop Water Requirement Calculator gives daily water use from a crop and a reference ET figure.

A Separate Rounded Imperial Example

The worked example on the calculator page is metric only. The example below is separate, worked in inches, and it is a rounded imperial example, not an exact conversion. It uses the same loam value of 17 percent, the same depletion of 50 percent, and the same efficiency of 80 percent.

Take loam with 12 inch roots. Root zone water is 0.17 times 12 inches, which is 2.04 inches. Net depth is 2.04 times 0.50, which is 1.02 inches. Gross depth is 1.02 divided by 0.80, which is 1.275 inches, shown as about 1.28 inches.

Common Mistakes

1. Applying net depth instead of gross depth. In the worked example the shortfall is 5.1 mm each time, because 25.5 mm at 80 percent efficiency stores only 20.4 mm.
2. Using gross depth to set the interval. The interval comes from net depth divided by daily ET, which is 25.5 divided by 5, or about 5 days.
3. Guessing efficiency instead of measuring it. A guess of 90 percent when the real figure is 70 percent leaves a gap of 8.1 mm at every watering.
4. Entering a root depth that does not match the real roots. A deep entry gives a long interval that the real roots cannot support.
5. Forgetting rain. Rain between waterings is water already applied. Subtract effective rainfall from the next irrigation.
6. Watering on a fixed calendar instead of the calculated interval. Daily ET changes with weather, so a fixed gap will be too long in a hot week.

When to Use the Calculator Again

Use the Irrigation Scheduling Calculator again whenever one of the inputs changes. Roots grow deeper through the season, so net depth grows with them. Daily ET rises in hot weather and falls in cool weather, so the interval changes with it. A change in system changes efficiency and so changes gross depth. Write the four numbers down each time: root zone water, net depth, interval and gross depth.

Frequently asked questions

Q: What is net depth in irrigation scheduling?
A: Net depth is the depth of water in the root zone that plants can use before the next watering. It equals root zone water times allowable depletion. In the worked example it is 25.5 mm.

Q: What is gross depth in irrigation scheduling?
A: Gross depth is the depth to apply at each watering so that net depth ends up stored after losses. It equals net depth divided by efficiency. In the worked example, 25.5 mm divided by 0.80 gives 31.9 mm.

Q: Why is gross depth larger than net depth?
A: Some applied water is lost before it reaches the root zone. At 80 percent efficiency, 20 parts in 100 are lost, so you must apply 31.9 mm to store 25.5 mm.

Q: What happens if I apply only the net depth?
A: The root zone ends short. Applying 25.5 mm at 80 percent efficiency stores only 20.4 mm, which is 5.1 mm short. The deficit repeats at every watering.

Q: How do I check the gross depth I applied?
A: Set a straight sided container or rain gauge on the soil surface, run the system for the planned time, and read the depth in millimetres. Compare it with the target of 31.9 mm, then lengthen or shorten the next run.