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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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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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.
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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:
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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.
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.
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For models specific to
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General models
More generally, the this project, please go here.
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.
Soil structure refers to the arrangement of soil particles (sand, silt, and clay) into stable units called aggregates, which give soil its structure. Aggregates can be loose and friable, or they can form distinct, uniform patterns. For example, granular structure is loose and friable, blocky structure is six-sided and can have angled or rounded sides, and platelike structure is layered and may indicate compaction problems.
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.
SWAT Maps
The combination of Soil, Water, And Topography variability within a field to guide irrigation decisions.
https://www.swatmaps.com/swat-water
Attributes of …
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Soil
Soil type
Soil texture
Field capacity
Available water (AKA Plant available water AKA “PAW”, AKA available water holding capacity or “awhc”): For peas, should be > 0.6
Maximum allowable depletion
(Permanent) Wilting point
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Crops
Crop coefficient
Daily water use
Crop stage
Field
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Lat / long
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Elevation
Permanent wilting point: 0.1442
Field capacity:
Growth stages
Support for project
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Alberta crop coefficients (Penman Monteith)
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Definition of zones
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Zone delineation strategies
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Other