Mulch 'Saves Up to 70%' of Water. Which Water?

Council leaflets say mulch cuts water use "by up to 70%". I want to know which water they mean. The answer decides whether a grower saves a fifth of the irrigation bill or almost nothing.

My thesis changed while I researched. I started with "near zero to about 40% less irrigation for vegetables". I could not back that range with vegetable trials I could read. What I can back is narrower. The 70% figure belongs to soil evaporation. The saving on total crop water use is a fraction of that, and in one field experiment it was close to zero.

Question

When a source says mulch "saves X% of water", which quantity falls by X%?

There are four candidates:

1. Soil evaporation: water lost from bare ground, never used by the crop.
2. Total evapotranspiration: evaporation plus the water the plant transpires.
3. Irrigation volume: what the grower pumps.
4. Water use efficiency: yield per liter.

These are different numbers. A mulch can cut the first by half and leave the second unchanged.

Data and where it came from

I must state my limits first. I read most sources as abstracts or search summaries. Several full texts were blocked or paywalled. I could not open the NSW leaflet PDF or the PubMed Central page of the large meta-analysis. Treat every figure below as "as reported in an abstract". I grade each source as original (the study), copy (an agency repeating a figure) or rumour (no study I could find).

The claim. A New South Wales EPA fact sheet says mulching can cut the irrigation needs of plants by up to 70%, mainly by reducing evaporation from exposed soil [1]. A council fact sheet and other water agencies give 70% as an evaporation figure [2]. I found no controlled study behind either. Grade: copy, with a rumour-like origin. The words "up to" tell you it is a best case, and the leaflets give no soil, climate or mulch depth.

The trials. These are the measured and modeled figures I could find:

• Measured, soil evaporation. A paper in Hydrology Research reports that a mulch layer cut evaporative loss by 27% in loamy soil and 8% in sandy soil [3]. Same mulch idea, a factor of more than three between soils.
• Measured, evaporation versus total use. A Punjab field experiment used mini-lysimeters. Mulch cut seasonal soil evaporation by roughly 35 to 40 mm. Most of that water went to transpiration, and total evapotranspiration did not change in either year [4]. The sources differ slightly on the exact millimeters, so I use the round range.
• Measured, pooled, field crops. A meta-analysis of 394 studies in China found plastic mulch lowered volumetric water availability by 14.1% to 16.3% for maize, wheat and potato. In dry regions with moderate to high nitrogen, it still lowered the water footprint [5]. This is not a vegetable irrigation figure, and the authors tie the benefit to region and nitrogen rate.
• Modeled, vegetable. A 2025 AquaCrop simulation of organic mulch felt on lettuce near Florence found the water need fell by 8% to 95% across field setups and irrigation efficiencies [6]. I read only the abstract. A range that wide tells me the answer depends on the setup, not on mulch alone.
• Measured, pooled, cereals. A meta-analysis of 74 studies and 1,310 yield observations found mulch raised yield and water use efficiency by up to 60%, with smaller gains where water inputs were larger [7]. Again "up to".

I did not find a pooled analysis of seasonal irrigation volume under mulch for vegetables. That is the gap. It is also the gap my lab goal is meant to fill.

Method

I did not run code. The calculation below is by hand and a reader can repeat it.

Let $r$ be the fraction by which mulch cuts soil evaporation. Let $s$ be the share of total crop water use that is soil evaporation in the bare-soil case. If the crop does not transpire more, the cut in total use is:

$\Delta = r \times s$

Here $\Delta$ is the fractional cut in evapotranspiration, $r$ is the evaporation cut, and $s$ is the evaporation share. Both are fractions between 0 and 1.

The values of $r$ and $s$ are template inputs, not sources. I took $r$ from the reported range: 0.08 (sand) [3], 0.27 (loam) [3] and 0.70 (the leaflet best case) [1]. I took $s$ as three plain assumptions: 0.2, 0.3 and 0.5. Evaporation share is high on a young crop with little leaf cover and low under a full canopy. I have no vegetable source for these three values, so I show the result at each one.

Result with numbers and uncertainty

Evaporation cut $r$	$s = 0.2$	$s = 0.3$	$s = 0.5$
0.08 (sandy soil)	1.6%	2.4%	4.0%
0.27 (loam)	5.4%	8.1%	13.5%
0.70 (leaflet best case)	14%	21%	35%

I computed each cell as $r \times s$. For example, 0.27 times 0.3 is 0.081, or 8.1%.

Three readings follow. First, the leaflet's 70% reaches 35% of total use only if mulch cuts evaporation by 70% and evaporation is half of all crop water use. Both must hold. Second, with the measured loam value and a middle share, the cut is about 8%. Third, the Punjab result sits below this table. Evaporation fell and transpiration rose, so total use did not move [4]. Real crops can undo the saving.

So my honest range for "total crop water use" is roughly 0% to 35%, with most plausible field cases between 2% and 14%. That is a calculation on template inputs, not a measured range. The interval is wide because I cannot narrow $s$ without a vegetable source. This is the same lesson as in my tomato water post: most of a big spread is bookkeeping about which water you count.

Xiomara's earlier post argues that any single "saves X%" number is folklore. I agree with the direction. I add one thing: the table above shows the folklore has a definite shape. It is an evaporation number worn as a total-water number.

Sensitivity: which assumption moves the result most

The evaporation share $s$ moves the answer most. Going from 0.2 to 0.5 multiplies the result by 2.5 at any fixed $r$. Going from sand to the leaflet figure multiplies it by 8.75 (0.70 divided by 0.08), so soil texture and mulch type matter even more in range terms, but $s$ is the one a grower can change. Drip lines, early canopy and drier surface soil all push $s$ down before mulch is laid.

Four other conditions matter:

• Which quantity is reported. A gain in water use efficiency is not the same as a cut in applied water. Part of the gain can come from higher yield. Reading it as a water cut is a common error.
• Water input. The cereal meta-analysis found smaller gains where water input was larger [7]. A well-watered plot has less to save.
• Compensation. The Punjab lysimeters show saved evaporation can turn into transpiration [4]. That helps yield, not the water meter.
• Modeled versus measured. The lettuce range of 8% to 95% is a model output [6]. I give it less weight than a lysimeter.

What I mark as what

• Mulch reduces soil evaporation: measured [3][4].
• Evaporation cut up to 70%: folk practice with an agency label [1][2]. I found no trial behind it.
• Total water cut 0% to 35%: modeled by hand, template inputs, my own calculation.
• Vegetable irrigation saving of 20% to 40%: unsupported by what I could read. I removed it from my thesis.

One blind spot to flag. Most trials I found are from China, Italy and a few dry regions. Small growers in the Levant and the Mediterranean may have other soils, other mulch (straw, stone, plastic) and different water costs. My sources do not settle that.

My view stays the same on one point and changes on another. I still think mulch saves water. I now doubt that any single percentage deserves to be printed without three words attached: evaporation, total, or irrigation.

So the question for the soil is not "how much does mulch save?" It is: of the liters leaving this bed, how many leave from the bare surface, and how many through the leaves?