Net Irrigation Depth Calculator

Calculator Inputs

Calculation Method

Choose a direct refill method or a management depletion method.

Field Capacity (%)

Permanent Wilting Point (%)

Current Soil Moisture (%)

Used when refilling the profile to field capacity.

Effective Root Zone Depth (cm)

Allowable Depletion, MAD (%)

Used for management-allowed depletion before irrigation.

Wetted Area Factor (%)

Use values below 100 for partial wetting systems.

Application Efficiency (%)

Irrigated Area (m²)

Crop ETc (mm/day)

Application Rate (mm/hour)

Example Data Table

Case	Method	FC %	PWP %	Current %	Root Depth cm	MAD %	Wetted %	Efficiency %	Net mm	Gross mm
Field crop sprinkler	Refill	32	16	22	60	50	100	80	60.00	75.00
Drip orchard block	Depletion	30	14	—	70	45	55	90	27.72	30.80
Vegetable bed	Refill	28	12	19	40	40	100	85	36.00	42.35

Formula Used

Refill Method

Net Irrigation Depth (Dn) = ((FC − θc) / 100) × Zr × 10 × Fw

Allowable Depletion Method

Total Available Water (TAW) = ((FC − PWP) / 100) × Zr × 10 × Fw

Readily Available Water (RAW) = TAW × (MAD / 100)

Net Irrigation Depth (Dn) = RAW

Supporting Calculations

Gross Depth = Dn / Ea

Net Volume = Dn × Area / 1000

Irrigation Interval = Dn / ETc

Operating Time = Gross Depth / Application Rate

Where:

FC = field capacity, percent volumetric moisture.
θc = current soil moisture, percent volumetric moisture.
PWP = permanent wilting point, percent volumetric moisture.
Zr = effective root zone depth in cm.
Fw = wetted area factor as a decimal.
MAD = management allowable depletion, percent.
Ea = application efficiency as a decimal.
Dn = net irrigation depth in mm.

How to Use This Calculator

Choose either the refill method or the allowable depletion method.
Enter field capacity, wilting point, and root zone depth values.
For refill mode, enter the current soil moisture value.
For depletion mode, enter the allowable depletion percentage.
Add wetted area, efficiency, area, ETc, and application rate.
Click the calculate button to see results above the form.
Review the graph, export the results, and compare scheduling values.

FAQs

1) What is net irrigation depth?

Net irrigation depth is the water depth needed inside the crop root zone to refill the soil moisture deficit. It excludes field application losses, so it represents the effective water the soil should actually receive.

2) How is net depth different from gross depth?

Net depth is the target water stored in the soil. Gross depth is the applied depth at the field surface after accounting for application efficiency, wind drift, runoff, deep percolation, and similar operating losses.

3) When should I use the refill method?

Use the refill method when you know current soil moisture and want to refill the profile back to field capacity. It is useful when you have sensor readings, gravimetric tests, or a recent soil moisture inspection.

4) When should I use the allowable depletion method?

Use the allowable depletion method when scheduling irrigation by management limits. It estimates net depth from total available water and the selected depletion fraction, even when current moisture is not directly measured.

5) Why is the wetted area factor important?

Drip and partial-area systems do not wet the entire soil surface. The wetted area factor reduces the effective soil volume considered in the calculation, preventing overestimation of the required irrigation depth.

6) What happens if current moisture is above field capacity?

If current soil moisture equals or exceeds field capacity, the refill deficit becomes zero. In that condition, the calculator reports that no refill irrigation is presently needed for the selected root zone.

7) How is irrigation interval estimated?

The interval is estimated by dividing net irrigation depth by daily crop ETc. It gives an approximate number of days between irrigations, assuming average weather, steady crop demand, and no major rainfall contribution.

8) Is this calculator suitable for every crop and soil?

It works well for many irrigation planning cases, but accuracy depends on reliable soil moisture, root depth, and crop data. Local field checks, soil tests, and agronomic guidance should still support final irrigation decisions.