An introduction to soil moisture and the bucket concept

Water is the mechanism through which nutrients are extracted from the soil and relocated to where the plant needs it most – roots, stalk, leaf, flower, and seed. Once present, water becomes the way in which roots can extend to access more moisture and additional nutrients found deeper in the soil. It is a balancing act to create the right moisture conditions that result in optimum growth, deeper roots, more moisture – thereby continuing the cycle. But how do we know if there’s enough moisture in the soil for plants to do this?

At a basic level, soil moisture can be thought of as the mix between soil and moisture (water) in soil. The following diagram describes a continuum from the most (left) moisture to the least (right).

When talking about soil moisture in a field, we’ll commonly use the concept of a “bucket” and how much water is in the “bucket”. When growing crops, it’s important to keep your fields above the wilting point - where the bucket is effectively empty from the perspective of a crop - and within the “Available water” zone. Above field capacity, you can think of the bucket as having a small hole - where any additional water added to the bucket drips out (is pushed deeper into the soil via gravity). There are many ways to measure the water in the bucket (see https://www.swatmaps.com/post/understanding-terms-used-to-discuss-soil-moisture-variability) that we won’t get into in this introductory article.

While at first this may seem like a fairly basic, there are actually many factors that impact:

How big the bucket is
How much water is put into the bucket
How water is taken out of the bucket

In many ways, the job of irrigation is to keep crops in the “Available water” goldilocks zone - that is above the wilting point but below the field capacity. Our job - as the LiteFarm team - is to understand a specific bucket and to help a farmer keep their bucket in its' goldilocks zone.

I’ve briefly outlined the most important of these below and we’ll explore them further later:

Soil texture: refers to the composition of the soil in terms of the proportion of small, medium, and large particles (clay, silt, and sand, respectively) in a specific soil mass. For example, a coarse soil is a sand or loamy sand, a medium soil is a loam, silt loam, or silt, and a fine soil is a sandy clay, silty clay, or clay. The amount of water available to the plants depends on the texture of the soil. Sandy soils retain much less water than loam soils. Loam soils are usually highly valued for their consistent crop production. Note that the greatest amount of crop available water is in the loam-to-silt loam texture. Understanding soil type variety in a given field is central to effectively maintain soil moisture levels in soils.

Topography: High spots and low spots will change how much water actually is actually absorbed into the soil locally versus running off to other areas. This applies to both precipitation and irrigation.
Electroconductivity (EC): is a convenient way to measure the amount of dissolved salts in the soil. Salt concentrations are important because they influence osmotic pressure. Water uptake by plants is regulated by the transpiration rate and the osmotic pressure in root cells. Water flows through root cell membranes from low-solute concentration to high solute concentration. Therefore, high salt concentration in the soil solution or in the water will reduce the amount of water available to crops. Secondarily, high electrical conductivity of the soil solution or nutrient solution can imply high concentration of particular ions, such as chlorides, sodium, and boron, which are potentially toxic to crops.
Crop type: The type of crop being grown on a field will heavily influence water needs. For example, some crops need relatively more water than others. Furthermore, different crops will root to different depths (rooting depths), changing what water is available to them. For example, in the image below any moisture in the 4 - 10m zone is inaccessible to Pimpinella saxifraga and Convolvulus trsgacanthoides, but is accessible to Pinus sylvestris and Zygophyllum xanthoxylo.

Crop stage: Crops will have different water (and nutrient) needs based on their stage of growth. This is easy to imagine by visualizing a freshly emerged corn stalk 1” tall versus a shoulder high stalk approaching maturity. These stages are well understood for most commercial crops and can be described in different ways. We’ll employ the idea of crop coefficients (Kc) and crop water use curve (such as the one below for dry peas).

Weather: Weather plays a huge, multifactorial role in effective irrigation. Not only can precipitation refill the bucket but things like temperature and windspeed can cause the bucket to drain more quickly. Knowing historical, present, and predicted future weather are important to keeping our bucket in the goldilocks zone.

Taking these inputs together, we can create a model to estimate for a given crop the amount of water that needs to be applied via irrigation in order to maintain our goldilocks zone.

Bucket size

As discussed above, many factors will influence the size of a bucket. These are, in order of influence:

Soil type
Topography

This video does a great job explaining the concept of water absorption versus run-off. It also starts to delve into how the presence of crops influences these processes. Topography affects water flow - in other words, where water sheds and where it collects. Soil texture and organic matter affect the total soil water holding capacity as well as actual plant available water (which are two different things!), so all three are working simultaneously to affect water and crop variability. The table below approximates the amount of available water by soil type. The gas tank metaphor used in this article is also helpful in understanding why different soil types have different amounts of available water.

Shown another way, as a table with easy to access, hard to access, and impossible to access categories:

Viewing the table and diagram, we can understand why sandy soils are less desirable for many types of agriculture than loamy, silty, and clay soil types.

Putting water in the bucket

Two activities can put water into the bucket:

Precipitation: Natural rainfall occurring during the growing season.
Irrigation: the process of manually or mechanically applying controlled amounts of water to the land.

Taking water out of the bucket

Many processes can take water out of the bucket:

Evaporation*: Vaporization of water on the surface of a plant or soil before it has a chance to be absorbed into the soil and used by crops. The primary inputs into evaporation are temperature, sunlight intensity and duration, and windspeed.
Transpiration*: the process whereby a plant absorbs moisture through its roots, uses it for growth and metabolism, and then releases it as evaporation from aerial parts, such as leaves, stems and flowers. The primary input into transpiration is crop and crop stage.
Runoff: The water that is not absorbed by the soil based on topography and other factors.
Groundwater recharge: When moisture above the “field capacity” level is pushed (by gravity) below the level where crops can effectively use it. Refer to the rooting depth visualization above.

Depending on the crops being grown and the ambient conditions, the “size” of the holes for each of these outflows may grow and shrink. For example, it’s easy to imagine that ET will increase on a very hot, very windy day (as compared to a warm, calm day) where a relatively larger proportion of moisture is lost to ET influences. It’s similarly easy to imagine that 1” of precipitation dropped during an intense 10 minute period would cause more runoff than the same amount of precipitation over a 2 hour period.

* In reality it’s fairly difficult to measure what moisture is lost to evaporation (movement of water to the air directly from soil, canopies, and water bodies) versus transpiration (movement of water from the soil, through roots and bodies of vegetation, and then into the air). Since we’re typically most interested in the aggregate amount of moisture taken out of the bucket, we’ll often describe these processes together as evapotranspiration (or ET). Evapotranspiration is an important part of the water cycle, and measurement of it plays a key role in agricultural irrigation and water resource management.

The above graph shows how plants transpire more as they mature. The increase in evaporation during the same period is likely due to higher temperatures. A quick preview of water use by crop stage (for dried peas, though generally similar across crops) is shown below:

Taken together

Below we can see a commercial offering showing what such a “water balance” system might look like.

On the x-axis we have time and on the y-axis we see the amount of moisture in the soil. As we might expect, several points on the y-axis denote the important levels (or derivatives of the levels) we discussed above:

Teal: Refill, AKA field capacity
Yellow: Safety, a point sufficiently proceeding the wilting point such that a farmer can schedule and perform irrigation before the crop becomes stressed and begins to wilt.
Red: Crop stress, AKA wilting point, where permanent losses to yield begin.
Brown: Permanent wilting point, where crops effectively have no access to moisture and die.

While this dashboard doesn’t explicitly show the reasons that soil moisture decreases over time, we can safely assume the four factors described in the previous section are the culprit. Indeed, we could likely correlate particularly hot or windy days with particularly precipitous drops in available water levels. Furthermore, based on when significant decreases in soil moisture begin (despite continued precipitation), we can assume the crop was planted in late April and shortly thereafter began actively evapotranspirating at an increasing rate until mid-Fall where the crop transitioned to a less active growth stage.

On the “filling the bucket” side of things, we see:

Dark blue: Rainfall events
Light blue: Irrigation events

As we might expect, the irrigation events clearly show a spike in soil moisture levels. Precipitation is a bit more muted but clearly serves to maintain high soil moisture prior to the growing season (where daily water use surpasses natural recharge from precipitation).

Modeling the state of the bucket and future water needs

Given in the importance of providing crops with adequate water levels during the growing season, you might imagine there are many models out there that attempt to provide farmers with insight into the state of their fields and whether (and how much) they need to irrigate.

Since some of the factors that go into modelling “the bucket” are regionally specific (e.g. soil types, sunlight hours, length of growing season, precipitation, avg. windspeed, etc.) many models aim to predict moisture levels for a specific area or region. For models specific to this project, please go here.

The Penman-Monteith equation seeks to model ET for general use.

When using this model, specific crop coefficients must be used to convert the reference evapotranspiration to actual crop evapotranspiration. Crop coefficients as used in many hydrological models usually change along the year to accommodate to the fact that crops are seasonal and, in general, plants behave differently along the seasons: perennial plants mature over multiple seasons, and stress responses can significantly depend upon many aspects of plant condition. A reference for variables used in Penman-Monteith equation is shown here. A collection of ET curves created by the FAO is available here.