An introduction to soil moisture and the bucket concept
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).
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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).
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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.
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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:
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Viewing this diagram, we can understand why loamy soils - with their relatively wide range of available water are so valued in agriculture while sandy soils are not.
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 applying controlled amounts of water to the land.
Taking water out of the bucket
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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.
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* 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 this 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.
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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.
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.
Alberta Irrigation Management Model: Key outputs from AIM:
Graphical and tabular reports of daily “year to date” moisture contiions, ET (crop water use), climate data, irrigation application amounts, surface run-off and deep percolation for any number of fields or sites within fields
Predictive assessment on crop water requirements and irrigation timing for designated near-future time periods
https://agriculture.alberta.ca/acis/imcin/irricast.jsp 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.
Models specific to Alberta
General models
More generally, the Penman-Monteith equation seeks to model ET for general use.
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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.
Scrap
Soil texture: 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.
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Soil porosity refers to the space between soil particles, which consists of various amounts of water and air. Porosity depends on both soil texture and structure. For example, a fine soil has smaller but more numerous pores than a coarse soil. A coarse soil has bigger particles than a fine soil, but it has less porosity, or overall pore space. Water can be held tighter in small pores than in large ones, so fine soils can hold more water than coarse soils.
Root zone
Evapotranspiration (ET) is a term used to refer to the combined processes by which water moves from the earth’s surface into the atmosphere. It covers both water evaporation (movement of water to the air directly from soil, canopies, and water bodies) and transpiration (movement of water from the soil, through roots and bodies of vegetation, and then into the air). Evapotranspiration is an important part of the water cycle, and measurement of it plays a key role in agricultural irrigation and water resource management.
ET Curves for Crops in general (FAO)
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Crop coefficients: To convert the reference evapotranspiration to actual crop evapotranspiration, a crop coefficient and a stress coefficient must be used. 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.https://agriculture.alberta.ca/acis/imcin/irricast.jsp
SWAT Maps
The combination of Soil, Water, And Topography variability within a field to guide irrigation decisions.
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