Mulch Water Saving & Cover the Soil, Save the Water

Crop water use has two parts: transpiration through the plant and evaporation straight off the soil surface. Mulch shades and covers the soil, so it mainly cuts the evaporation part. On young or widely spaced crops a lot of bare soil is exposed, so the evaporation share is large and the saving from mulching is biggest. The plant's transpiration carries on as normal.

Saved depth per day = ET × the soil-evaporation share × the fraction of evaporation the mulch cuts. For example 5 mm/day × 0.30 evaporation share × 0.70 cut ≈ 1.05 mm/day saved. Multiply the saved depth by the area over the period to get the total volume — 1 mm of water over 1 m² is 1 litre, so 1 mm over a hectare is 10 m³.

It's the proportion of total crop water use that leaves as evaporation from the soil rather than transpiration through the plant. It is high (often 0.3–0.5 or more) when the canopy is open — early growth, wide rows, orchards — and low under a full closed canopy that already shades its own soil. The bigger the bare-soil exposure, the more mulch can save.

A good organic mulch (straw, crop residue) or plastic film can reduce soil evaporation by roughly 50–80% while it stays intact and covers the surface. Thicker, lighter-coloured and well-laid mulch cuts more; thin, patchy or decomposed mulch cuts less. The tool lets you set the cut so you can compare mulch types and depths.

The benefit is largest on young or open crops in hot, dry, windy weather, when bare soil loses the most water and the canopy gives little shade. As the canopy closes the soil is already shaded and the extra saving falls. Mulch also suppresses weeds and moderates soil temperature, which add value beyond the water saving alone.

Mostly it reduces the wasted evaporation, not the productive transpiration the crop needs to grow. So you irrigate less to deliver the same water to the roots, raising water-use efficiency. In very hot conditions mulch can also slightly cool the root zone and reduce stress, but the headline benefit is cutting evaporation losses.

Plastic film, especially when it seals the bed, can cut evaporation more and warms the soil, which suits early or high-value crops; but it costs more and needs disposal. Straw and crop residue are cheaper, add organic matter and keep the soil cooler, with a slightly lower but still large evaporation cut. Both should be kept off the stems to avoid rot.

Spread organic mulch evenly to a good depth over moist soil, keeping it a little away from stems and trunks so they don't stay wet and rot. For plastic film, lay it tight over a well-prepared bed with the edges anchored, and plant through holes. Top up organic mulch as it breaks down so the cover — and the saving — is maintained.

Yes, and the two work well together. Drip already wets only a small area and limits evaporation; adding mulch over the wetted strip cuts the remaining surface loss further and holds moisture between pulses. Combine the saving here with the Drip Water Saving and Water Use Efficiency tools to plan a very water-efficient system.

They're a planning estimate. The real saving depends on crop, canopy, climate, mulch type, depth and condition, and how much of the soil is actually covered. Use realistic values for the evaporation share and the cut, treat the result as a guide for the season, and adjust as the canopy closes and the mulch ages.