Water-Related Land Use
Why
The question of why we need this dataset is central to what we do and drives continuous improvement within this dataset. As you may have guessed, the "why" comes down to one crucial resource, water.
Understanding our water use is important for many reasons. In its most simple terms, we all need water. Water is needed for the ecology of the natural landscape and the wildlife that relies on it. Water is needed for agriculture, to supply industry, and for our everyday life.
Water can be discussed in many ways, but it often comes down to legal terms. There are legal compacts between states and countries, and water rights that define where and how much water can be used.
Here at the Division of Water Resources, we are continually brainstorming regarding our water use and supply. This drives two primary questions:
- How much water is Utah using?
- How much water will Utah need in the future?
Within these questions lie countless complexities, however, here we will focus on water-related land use.
Understanding land use and where water is applied is critical in answering these questions. This helps inform where and what the water is being used for.
Where the water is being used influences several variables that contribute to answering the two primary questions.
For agricultural purposes, the where influences climatic variables such as humidity and solar radiation. These variables influence how fast plants transpire and how fast water evaporates. Together these terms are referred to as evapotranspiration (ET).
What is not used by the plants and consumed via ET is either returned to surface water and reused downstream or recharges groundwater supplies. This unused water is referred to as return flow.
Agricultural practices can differ in how water is used. For example, two of the most common irrigation techniques are flood and sprinkler irrigation. Flood irrigation generally applies more water to the field, leaving more excess water to runoff, or percolate into the groundwater. While sprinkler irrigation generally applies water more precisely and a higher percentage of the water is used by the crop.
Understanding how agricultural practices evolve over time can give insight into how much water will be available. An easier change to understand is dry land converted to sprinkler irrigation.
As you can see, center-pivot irrigation was implemented on previously dry land. Using this data, we can inform our water budget that these plots are now irrigated and what crop is being grown. In this case, the area is likely using more water.
While irrigation does come online, irrigation also goes offline. In the above example, you can see existing pivots that may not have been irrigated that year. However, sometimes, there are rotations between plots, and these yearly changes are important to track. While one center-pivot may have begun irrigating, maybe another stopped irrigating for that season. Land use can change in many ways, and sometimes water intensity or the way water is used can change. Below you can pan across the imagery and see agricultural land convert to urban. Left (2014), Right (2021)
Water is not used for the fields that no longer exist. However, urban environments do use water for watering lawns, showers, washing dishes, and more. These types of uses are tracked with our Municipal and Industrial (M&I) Water Use dataset.
The crops that are planted also change. Each plant species uses water differently, some transpire water rapidly, while others utilize water more slowly. These differences drive the need to track the crops planted so that the crop's ET rate can be estimated. Winter wheat shown in this picture is the fifth most common crop type in Utah as of 2021, representing ~52,000 acres of irrigated agriculture. Winter wheat has a mean annual ET rate of 22.6 inches.
Crops change for a variety of reasons, sometimes a certain crop is in more demand than others, sometimes a plot is allowed to sit fallow during dry seasons, low demand, or to allow the soil to rest. Crops are rotated to legumes, such as the alfalfa in this picture, to naturally add nitrogen to the soil. There are countless reasons crops may change, and tracking this allows for more precise water use estimates.
Alfalfa is the most common crop throughout Utah, representing ~783,000 acres of irrigated agriculture in 2021. Alfalfa has a mean annual ET rate of 28.9 inches. You can imagine how crop choice can impact water use.
How
The way these data have been collected has changed over time, a process that started in 1985. Initially, the entire state was surveyed by field crews over a ten-year and later a six-year period. We now use other methods to produce a yearly dataset.
This process begins each summer with a brief field season. We no longer survey the entire state, and in 2021 3,247 fields were checked representing ~1% of the data.
The vast majority of this work harnesses GIS processes to complete the yearly dataset. The primary sources of data for these processes are aerial imagery from the USDA National Agriculture Imagery Program (NAIP), crop data from the USDA Cropland Data Layer (CDL), and historic data from our own Water-Related Land Use (WRLU) data.
NASA Satellites Keep Watch on U.S. Food Supply
HOW
The data produced covers the entire state of Utah along with portions of the surrounding states that influence Utah's water.
HOW
In this process, we attempt to represent all land that influences agricultural water.
HOW
We assign an irrigation method to each "polygon" that represents a field or a portion of a field. Land use is an important aspect in all of these questions. Agriculture uses a large percentage of water, however, not all of this water is lost and can return to the stream or infiltrate into the groundwater. Part of figuring out what is lost is understanding ET. To get this we need to understand what crops are being planted
HOW
As you can see, we also assign each polygon a crop. Since we have a pretty good idea of what each crop is and if the field is irrigated, we can estimate the amount of water that is lost through ET. These estimates are made using the culmination of countless studies measuring different crop's ET, and physically based equations supported by the scientific community. With remote sensing, the inputs to these equations are improving and provide more information to manage our water resources.
What
With these data, we can track many attributes that may influence water use, and these attributes can be used to budget our water and make projections into the future. For example, below we can see the slope from a linear model showing the percent change (2017-2021) between irrigation methods per hydrologic basin.
We can see flood irrigation is fairly static or losing irrigated acreage in many basins across the state. We can also see sprinkler irrigation and drip irrigation are becoming more common. This will change how water is being used and influence local hydrological cycles and water loss.
Different crops are associated with different ET values, as some crops are more water intensive. Tracking these trends is important for understanding our current and future water needs. Below we can see the slope from a linear model showing the % shift between crops associated with lower or higher ET values between 2017 and 2021. The ET intensity generally trended towards more water intensive crops.
Many trends can be seen in these data that are important to understand when planning for our future. Explore the data below with the interactive dashboard. By clicking any of the plots or basins you can see a sub-selection of the data. For example, click on Alfalfa in the pie-chart. We can see this is generally irrigated agriculture, which mainly uses sprinkler irrigation. Alfalfa is generally becoming a more common crop. Interestingly in 2015, there was a large drop in the acreage of alfalfa planted. What are some trends that you see?
Links
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