The Soil is Cracking: Satellite Drought Maps

Gülüzar Yiğit
Oct 7
4 min read

Drought doesn't begin overnight. First, rainfall decreases, then humidity decreases, and plants lose their color. The key to understanding this silent progression is not just looking at the ground, but looking from above. Satellites have been tracking these changes for years. In this article, I'll explain how atmospheric irregularities are mapped, what data helps us predict soil drying, and why we should think of drought as a matter of "foreknowledge."

What Does Drought Tell Us?

Drought isn't just a farmer's problem. It could explain why tomatoes on the supermarket shelf are twice as expensive this year, why water is cut off more frequently, and even why showers are interrupted that day. If the soil is cracking, it means there's an imbalance somewhere. Drought is the earth's reflection of this atmospheric deterioration. But this crisis isn't so one-sided.

Sometimes it doesn't rain: we call this a meteorological drought. Sometimes it rains but the soil doesn't hold: we call this an agricultural drought. If dams are full but rivers are receding, we call this a hydrological drought. Drought isn't just a natural phenomenon; it's a multilayered problem with economic, social, and ecological impacts. Indeed, since the 1900s, 11 million people have lost their lives due to drought, and 2 billion people have been directly affected.

What is the Drought Map Note for the Ergene Basin?

Drought analyses aren't just theoretical. For example, in a study conducted in the Ergene Basin, NDVI, LST, precipitation, and soil moisture data were combined. These data produced four different indices: VCI, TCI, PCI, and SMCI. These data were then combined using the PCA (Principal Component Analysis) method to produce the CDI (Combined Drought Index) map.

Monthly average drought maps for the Ergene Basin represent the months of May (a), June (b), July (c), August (d), September (e), and October (f), respectively. The color scale indicates increasing drought severity from green to red:

Dark green : No drought. Light green : Mild drought. Yellow : Moderate drought. Orange : Severe drought. Red : Extreme drought. 2001 was a disaster for the region: 65% of the basin was dry, and 11% was extremely dry. This map isn't just a scientific document; its 79% correspondence with sunflower yields demonstrates its realistic nature.

As summer approaches (especially between July and September), the red and orange zones increase significantly. This indicates that the risk of drought peaks during the period when agricultural activities are most intense. September stands out on maps as the month with the highest incidence of extreme drought. June and May are relatively greener, meaning less risky.

The maps in Figure 2 answer the question of how drought changes seasonally in the Ergene Basin and how closely remotely sensed drought maps (CDIs) match conventional indicators. The first row of maps shows the average drought severity for each month from May to October. The color scale from green to red reveals both the extent and intensification of the drought over time. The bottom row of maps shows the relationship between the CDI and the SPI and SPEI indices. The highest agreement was achieved with indicators calculated at one-month scales. This demonstrates that the combined drought maps generated by satellites largely match the field data and can be used effectively in early warning systems.

These visuals not only make it possible to visually track the seasonal drought cycle, but also provide timing-based early warning and planning support for local governments and farmers.

What Does It Take to Map Drought?

Drought with Remote Sensing Technology

Satellites observe the Earth without missing a single pixel. Satellites like MODIS, Landsat, and NOAA-AVHRR provide data on land health (NDVI), surface temperature (LST), plant water content (NDMI), and soil moisture. These data directly reflect the effects of drought.

Frequently Used Indicators:

NDVI: Health of green vegetation
NDMI: Plant water content
LST: Surface temperature (thermal stress)
VCI/TCI/PCI/SMCI: Stress indicators derived from the above data

These datasets can be analyzed separately, but the most powerful analyses are achieved when used together. For this purpose, engineers use PCA. This method statistically simplifies multiple datasets and presents the most meaningful combined data as a CDI map.

What Do SPI and SPEI Do?

The most commonly used indicator in Türkiye is the SPI. It looks only at precipitation data. It's easy to calculate and clear to interpret. The SPEI, on the other hand, takes into account temperature and evaporation in addition to precipitation. Both operate on different time scales:

1 month: Meteorological drought
3–6 months: Agricultural drought
12+ months: Hydrological drought

How Are Drought Maps Made in Türkiye?

The General Directorate of Meteorology (MGM) publishes SPI-based drought maps every month. These maps cover timeframes of 1, 3, 6, 9, and 12 months. The January 2024 map includes a severe drought warning for Eastern and Southeastern Anatolia. These maps provide guidance not only to the public but also to municipalities, farmers, and energy planners.

So, is it reliable?

Yes. The SPI is the standard methodology recommended by the WMO (World Meteorological Organization). However, it only reflects meteorological drought. Additional data is required for agricultural and hydrological impacts. However, when combined with indices like the CDI, it becomes a powerful warning system for both agriculture and water management.

Drought doesn't just crack the soil. It directly impacts food prices, water resources, and quality of life. But if we detect it early, we can mitigate the damage. We monitor it with satellites, measure it with indices, and report it with maps. The key is to hear what those maps say in a timely manner.

In this article, we discussed not only what drought is but also how it's monitored. From data and maps to indicators and warning systems, there's a significant scientific effort behind many of these steps. The studies below form the foundation of this article and support my arguments.

Source

Gümüş, KA, Balçık, FB, Esetlili, T., & Kahya, C. (2023). Monitoring drought dynamics using remote sensing-based combined drought index in Ergene Basin, Turkey. Open Geosciences, 15 (1), 1–14. https://doi.org/10.1515/geo-2022-0594
Öztürk, YD, & Ünlü, R. (2022). A review of drought analysis studies in Turkey. Journal of Disaster and Risk , 5 (2), 126–144. https://doi.org/10.35341/afet.1124880
General Directorate of Meteorology. (2024). Drought Analysis. https://www.mgm.gov.tr/veridegerlendirme/kuraklik-analizi.aspx
Energy Economics. (2024). Drought Maps Published. https://www.enerjiekonomisi.com/kuraklik-haritalari-yayinlandi-mgm/31754