{
    "name": "Soil Water Deficit Calculator",
    "url": "https://calculators.geekmidst.com/calculators/soil-water-deficit-calculator",
    "markdown": "https://calculators.geekmidst.com/calculators/soil-water-deficit-calculator.md",
    "category": "Agriculture",
    "description": "Find how many millimetres of water the root zone needs to return to field capacity from a soil moisture reading, and how depleted it is.",
    "summary": "The soil water deficit is the gap between field capacity and current moisture, multiplied by root depth. It is the net irrigation needed to refill the root zone.",
    "also_called": [
        "Soil moisture deficit calculator",
        "Root zone depletion calculator"
    ],
    "applies_to": "Worldwide",
    "last_updated": "2026-10-29",
    "computation": "Results are computed in the visitor’s browser on the HTML page. This API describes the calculator; it does not compute results.",
    "inputs": [
        {
            "name": "basis",
            "label": "Moisture readings are",
            "type": "segmented",
            "default": "volumetric",
            "options": {
                "volumetric": "Volumetric %",
                "gravimetric": "Gravimetric %"
            }
        },
        {
            "name": "bulk_density",
            "label": "Soil bulk density",
            "type": "number",
            "default": "1.4",
            "unit": "g/cm³",
            "shown_when": {
                "basis": "gravimetric"
            }
        },
        {
            "name": "fc",
            "label": "Field capacity",
            "type": "number",
            "default": "32",
            "unit": "%"
        },
        {
            "name": "current",
            "label": "Current moisture",
            "type": "number",
            "default": "24",
            "unit": "%"
        },
        {
            "name": "wp",
            "label": "Wilting point",
            "type": "number",
            "default": "14",
            "unit": "%"
        },
        {
            "name": "root_depth",
            "label": "Root depth",
            "type": "number",
            "default": "0.6",
            "unit": "m"
        },
        {
            "name": "area",
            "label": "Area",
            "type": "number",
            "default": "1"
        },
        {
            "name": "area_unit",
            "label": "Area in",
            "type": "segmented",
            "default": "ha",
            "options": {
                "ha": "Hectares",
                "acre": "Acres"
            }
        }
    ],
    "outputs": [
        {
            "key": "deficit_mm",
            "label": "Water deficit",
            "format": "number"
        },
        {
            "key": "depletion_pct",
            "label": "Share of available water used",
            "format": "percent"
        },
        {
            "key": "available_mm",
            "label": "Available water left",
            "format": "number"
        },
        {
            "key": "taw",
            "label": "Total available water",
            "format": "number"
        },
        {
            "key": "volume_m3_ha",
            "label": "Water to refill per hectare",
            "format": "number"
        },
        {
            "key": "volume_m3",
            "label": "Water to refill the area",
            "format": "number"
        }
    ],
    "formula": {
        "text": "Deficit (mm) = 1000 (θFC − θ) Zr;  TAW = 1000 (θFC − θWP) Zr;  Depletion = Deficit ÷ TAW;  θ (vol) = θ (grav) × Bulk density",
        "variables": {
            "θ": "volumetric water content (m³/m³)",
            "Zr": "root depth (m)",
            "Bulk density": "g/cm³, to convert gravimetric readings"
        }
    },
    "worked_example": {
        "inputs": {
            "basis": "volumetric",
            "bulk_density": "1.4",
            "fc": "32",
            "current": "24",
            "wp": "14",
            "root_depth": "0.6",
            "area": "1",
            "area_unit": "ha"
        },
        "steps": [
            "Deficit = 1000 × (0.32 − 0.24) × 0.6 = 48 mm.",
            "TAW = 1000 × (0.32 − 0.14) × 0.6 = 108 mm, so 48 ÷ 108 = 44.44% is used.",
            "Refill volume = 48 mm × 10 = 480 m³ per hectare."
        ],
        "result": "The root zone is 48 mm short of field capacity (480 m³/ha), with 44% of its available water used."
    },
    "interpretation": "Compare the depletion with your crop’s allowable depletion (often 30–60%). When it is reached, apply the deficit divided by your application efficiency.",
    "assumptions": [
        "The moisture reading represents the whole root zone.",
        "Water density is 1 g/cm³ when converting gravimetric readings."
    ],
    "limitations": [
        "Single-layer estimate; layered soils need a layer-by-layer sum."
    ],
    "rules": null,
    "faqs": [
        {
            "question": "Volumetric or gravimetric?",
            "answer": "Most soil moisture probes read volumetric %. Oven-dry samples give gravimetric %; multiply by bulk density to convert."
        },
        {
            "question": "Should I apply the whole deficit?",
            "answer": "Apply the deficit plus losses. Applying more than the deficit drains below the roots and can leach nutrients."
        }
    ],
    "sources": [
        {
            "title": "FAO Irrigation and Drainage Paper 56, Chapter 8: soil water availability",
            "url": "https://www.fao.org/3/x0490e/x0490e0e.htm"
        }
    ],
    "related": [
        {
            "name": "Irrigation Scheduling Calculator",
            "url": "https://calculators.geekmidst.com/calculators/irrigation-scheduling-calculator"
        },
        {
            "name": "Drip Irrigation Discharge Calculator",
            "url": "https://calculators.geekmidst.com/calculators/drip-irrigation-discharge-calculator"
        }
    ],
    "disclaimer": null,
    "publisher": "GM¹X CalcVerse"
}