Using the Bureau of Land Management Drought Reports

The video embedded below and the accompanying article were developed to support BLM Staff in using the BLM Drought Reports available through the BLM Climate & Remote Sensing Data Reports at reports.climateengine.org.

Drought is a primary driver of vegetation production, water availability, and wildfire risk on BLM-managed lands and the agency is increasingly prioritizing awareness of drought conditions in decision-making. However, the complexity of drought impacts and the size of drought datasets makes selecting relevant drought indicators at the appropriate scale for planning and adaptive management decisions challenging for resource managers. To overcome these challenges, Climate Engine has partnered with the BLM and NOAA's National Integrated Drought Information System (drought.gov) to produce consistent, up-to-date, and accessible drought reports for all BLM-managed lands. In this article, we discuss the new website and walk through the drought reports so that you can start using them.

About the BLM Drought Reports

The BLM Climate & Remote Sensing Data Reports website is located at reports.climateengine.org and provides near real-time PNG and PDF drought reports at the scale of BLM state offices, district offices, field offices, and grazing allotments in the Western U.S. The reports use two Multi-Indicator Drought Indices, developed in conjunction with NOAA’s National Integrated Drought Information System (NIDIS) (Drought.gov), that capture different types of drought. The tool is specifically intended to provide timely high-resolution drought information at the scale of BLM-managed land units in an easy-to-use website to streamline decision-making and reporting.

Visit the BLM Drought Reports though the BLM Climate & Remote Sensing Data Reports website at reports.climateengine.org

Whenever possible, we recommend using these reports alongside additional lines of evidence to demonstrate drought impacts and account for local conditions, such as field visits; field vegetation data; satellite vegetation data; soil moisture, stream gauge, and groundwater level dat; photo points; and other drought indicators.

Before diving into the reports in more detail, let's explore a few core concepts of drought and numerical drought indices and discuss the Multi-Indicator Drought Indices in more detail.

Drought on BLM-managed lands

Drought is a complex slow-onset hazard that is difficult to represent on a single map or using a single indicator. Meteorological variables such as precipitation, solar radiation, windspeed, and humidity as well as factors such as the duration and severity of drought conditions and the response time of the resource of interest are all important for identifying appropriate drought metrics.

Programs like the U.S. Drought Monitor provide consistent and timely drought information in an easy-to-use format but can lack spatial detail at the scale of BLM decisions. In those cases using additional drought indices can aid in a thorough drought assessment.

A significant challenge in characterizing drought impacts is that different resources respond differently to drought. For example, drought duration is an important factor in drought assessment, with hydrologic resources such as groundwater and reservoirs being more impacted by long-term drought and vegetation being impacted by short- to medium-term drought. To characterize drought impacts on different resources, it is helpful to think about the different types of drought.

Types of drought

The climatological community has defined different types of drought:

Meteorological drought is when dry weather patterns dominate an area.
Agricultural drought is caused by conditions that result in adverse plant response such as reduced crop and forage yields to total crop or forage failure.
Hydrologic drought is characterized by a lack of water in the hydrological system, manifesting itself in abnormally low levels in rivers, streams, lakes, aquifers, and reservoirs.
Socioeconomic drought is associated with the supply and demand of economic goods with elements of meteorological, agricultural, and hydrologic drought.
Ecological drought is when natural ecosystems are affected by drought.

Considering the different types of drought in the context of the drought decision being made can be helpful for selecting appropriate drought indicators. We can represent these different types of drought using quantitative meteorological data to calculate drought indices, which account for drought severity and duration.

Drought indices

There are several different drought indices that incorporate data from a variety of sources including climate datasets, soil moisture estimates, and satellite-based vegetation indices. Some metrics like the US Drought Monitor also use expert input to develop their maps.

Some of the most useful and flexible drought metrics are multiscalar drought indices, which are a common set of drought indices used to represent drought severity, location, timing, and duration. Multiscalar drought indices are especially useful because they can be applied over any timescale, ranging from 14 days to 5 years or more, which means they are flexible and can be used to represent the different types of drought, which we discussed in the previous section.

Three common multiscalar drought indices are the Standardized Precipitation Index (SPI), the Standardized Precipitation Evapotranspiration Index (SPEI), and the Evaporative Demand Drought Index (EDDI). Each of these multiscalar drought indices are calculated using similar statistical methodologies, which we will discuss below, but with SPI representing only precipitation, EDDI representing only evaporative demand, and SPEI representing both precipitation and evaporative demand.

Multiscalar drought indices such as Standardized Precipitation Index (SPI), the Standardized Precipitation Evapotranspiration Index (SPEI), and the Evaporative Demand Drought Index (EDDI) can be calculated for time periods between 14 days and 5 years or more, which makes them flexible as indicators for the different types of drought.

Another common drought index is the Palmer Drought Severity Index (PDSI), which accounts for both precipitation and evaporative demand. PDSI is not a multiscalar drought index because it is calculated over a fixed time period of about 9 months.

Calculating multiscalar drought indices

Each of the multiscalar drought indices is calculated as a standardized anomaly meaning that we fit a distribution to the data and compute the percentile of the value. See an example of calculating a 90-day Evaporative Demand Drought Index (EDDI) for August 30, 2023 to better understand how we calculate multiscalar drought indices.

Using gridMET to calculate drought indices

We calculate the multiscalar drought indices using two datasets from the University of California Merced, gridMET and gridMET Drought. Together, these datasets provide high-resolution multiscalar drought indices that are produced every five days. We provide more information about gridMET here and gridMET Drought here.

The gridMET Drought dataset uses high resolution climate and surface balance datasets that provide daily estimates of precipitation and evaporative demand in order to calculate a suite of multiscalar (SPI, SPEI, EDDI) drought indices every five days.

Using and interpreting multiscalar drought indices

To develop some intuition about how multiscalar drought indices represent different drought timescales, let’s look more closely at SPI over two time periods in Nevada during the 2022-2023 water year.

The 2022-2023 water year was pretty exceptional in Nevada. In the map of precipitation anomalies below, we can see that nearly the entire state had above-average precipitation compared to the average in 1991-2020 with some locations having up to 10 inches more than average. In the time-series figure below, we can see when precipitation occurred throughout the year, with several wet periods in the first half of 2023 and several other wet events in the late summer.

Left: Map of 2022-2023 water year precipitation anomaly relative to 1991-2020 average in Nevada. Right: Time-series figure of precipitation during the 2022-2023 water year in Nevada.

Let’s compare the 90-day and 270-day SPI when overlaid onto the precipitation data. In the 90-day SPI plot, we can see that the rainy period in December and January dramatically increased SPI and that the event in August (Hurricane Hilary) also increased SPI. When we now look at 270-day SPI we can see these same events also increased the SPI value, but that changes in 270-day SPI are more gradual and the line is smoother. That is because the 270-day SPI accounts for precipitation during the entire 270-day period preceding the date of interest, so only prolonged or severe dry or wet periods will move the value.

90-day SPI overlaid on precipitation in Nevada for the 2022-2023 water year

270-day SPI overlaid on precipitation in Nevada for the 2022-2023 water year

Now that we have some understanding and intuition about multiscalar drought indices, we can return to the Multi-Indicator Drought Indices in the new BLM Drought Reports.

The Multi-Indicator Drought Indices

The Multi-Indicator Drought indices used in the new BLM Climate & Remote Sensing Data Reports were developed in collaboration with NOAA NIDIS and represent two different drought timescales, short-term and long-term periods.

The Multi-Indicator Drought Indices look at current drought conditions across the U.S. by integrating several key drought monitoring indices into a single objective, computer-generated map. The example shown below provides a comparison between the U.S. Drought Monitor and the Multi-Indicator Drought Blends and we can see that the drought blends often provide more spatial detail, such as representing drought gradients at the scale of individual mountain ranges.

The Multi-Indicator Drought Blends are a composite of multiple multiscalar drought indices that represent long-term and short-term drought at the scale of BLM land units.

Multi-indicator or composite indices combine several of the drought indices that we introduced in the previous section. The Short-Term Drought Blend represents periods from 30 days to 9 months and the Long-Term Drought Blend represents periods from 6 months to 5 years.

Below are the formulas used to calculate each of the drought blends used in the reports. You will recognize the drought indices such as SPI and PDSI that were introduced earlier in this article. Together, the Long-term Drought Blend and Short-term Drought Blend represent two drought timescales that can be useful in drought assessment and can help inform drought response.

The Multi-Indicator Drought Blends combine were developed in conjunction with NOAA NIDIS and are based off of research by NOAA's Climate Prediction Center. They combined SPI, PDSI, and Palmer's Z-Index at different timescales to represent long-term and short-term drought processes.

It’s worth reiterating that no single drought indicator can characterize all types of drought and that referring to streamflow, groundwater, snowpack, vegetation, and other datasets that are relevant to the resource of interest will help ensure a thorough drought assessment

More information about the Multi-Indicator Drought Blends can be found here.

BLM Drought Reports

Now that we have an understanding of drought processes and drought indices, let’s dive into the BLM Drought Reports on the BLM Climate & Remote Sensing Data Reports website at reports.climateengine.org. To see the drought reports, click on the Drought option from the menu.

New drought reports are produced every five days for each BLM state office, district office, field office, and grazing allotment. Let’s walk through an example using a grazing allotment in Colorado’s Uncompaghre Field Office.

By default, the most recent drought report will be displayed, but you can find an archive of recent reports by using the "Date" dropdown menu. In this example, we'll display the drought report for December 20, 2023. Next, we’ll select the Colorado State Office, then the Southwest District Office, next the Uncompahgre Field Office, and finally the Mesa Creek grazing allotment. Notice that every time a new land unit is selected, the corresponding report is displayed.

Make the selections above to produce the report for Mesa Creek Allotment on December 20, 2023 shown in the section below.

The scale of the decision being made or the scale of the desired assessment will inform whether a broad- or local-scale report — state office, district office, field office, or grazing allotment — is most appropriate. For example, the field office scale may be relevant for identifying areas that may be experiencing drought for staff to investigate further or the grazing allotment scale may be relevant for developing grazing plans or performing Land Health Assessments.

Drought Report sections

Within each report there are three primary data summaries, 1) the current conditions maps, 2) the summary table, and 3) the time-series figures.

In the Mesa Creek Allotment, the Long-term Drought Blend map indicates moderate to severe drought across the allotment, while the Short-term drought blend indicates abnormally dry to moderate drought conditions.
The summary table provides area estimates of the amount of the land unit in different drought classes for the current date, 3 months ago, and one year ago. For both the Short-term and Long-term drought blends we can see that the allotment is currently in a more severe drought than it was 3 months ago or one year ago.
The time-series figures provide context for how drought has progressed since 1985. For this allotment we can see that prolonged long-term drought occurred in 2013 and then consistently in the late 2010s and early 2020s. Using these figures we can better understand how current drought conditions compare to previous events.

Each report has three primary data summaries, 1) the current conditions maps, 2) the summary table, and 3) the time-series figures.

Each report is produced for download as a PNG and PDF. For example, to download a PDF file for a given land unit, simply select PDF as the report format and click download.

All reports are available as one-page PNG image files and PDFs.

Additional BLM reports

We’re producing other reports using a common methodology which can also be accessed at reports.climateengine.org. For example, the Site Characterization reports provide satellite-based vegetation data from the Rangeland Analysis Platform (rangelands.app) and climate data from gridMET for every BLM state office, district office, field office, and allotment and are updated yearly.

The Site Characterization reports are currently in review and are receiving updates. Below is an example of the current reports for the Arizona Strip Field Office.

The Site Characterization reports are currently in active development and provide yearly summaries of vegetation cover and production using the Rangeland Analysis Platform (rangelands.app) and for climate and drought from gridMET.

Using the Climate Engine app for advanced drought assessment

The multi-indicator drought blends and many other multiscalar drought indices can be calculated in the Climate Engine app at app.climateengine.org. The Climate Engine app has been supported by the BLM since 2015 and makes mapping and analyzing dozens of drought indicators easy and fast. It also has satellite-based vegetation datasets like the Rangeland Analysis Platform and RCMAP available, which can be powerful for assessing drought sensitivity and drought impacts on vegetation.

The Climate Engine web app is built using the Google Earth Engine cloud computing platform. It provides a no-code user interface for analyzing more than 80 datasets and hundreds of climate and vegetation variables.

Visit climateengine.org and our Support Site at support.climateengine.org for more information about the Climate Engine app.