📄 earth_science

Hydroclimatic Variability and Drought Characterisation in Northern Togo, 1981-2025

This study reveals that while rainfall trends in northern Togo (1981–2025) remain statistically insignificant, a significant warming trend since the early 2000s has intensified evapotranspiration and moisture deficits, necessitating temperature-sensitive drought indices for effective monitoring and resource management in this climate-vulnerable region.

Original authors: Malaguièhèwa Grâce Pekeyi, Appollonia Aimiosino Okhimamhe, Tayo Iyanda Yahaya, Isaiah Sule

Published 2026-07-08

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CC BY 4.0

Original authors: Malaguièhèwa Grâce Pekeyi, Appollonia Aimiosino Okhimamhe, Tayo Iyanda Yahaya, Isaiah Sule

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

The Big Picture: A Thirsty Region

Imagine Northern Togo as a giant garden. For decades, people have been watching the rain to see if the garden will grow. This study, covering the years 1981 to 2025, looked at three specific "neighborhoods" in this garden: Sokodé, Kara, and Mango.

The researchers asked a simple question: Is the garden getting wetter, drier, or staying the same?

The Plot Twist: It's Not Just About Rain

Usually, when we think of a drought, we think of "no rain." But this study found a sneaky twist.

The Rain Gauge (What we expected): If you look strictly at how much rain fell, the garden hasn't really changed much. The amount of rain is roughly the same as it was 40 years ago. There is no clear trend of it getting significantly wetter or drier.
The Thermostat (The hidden problem): However, the air has gotten noticeably hotter. The temperature has been rising steadily, like a slow-burning fire under a pot of water.

The Analogy: Imagine you have a cup of water (the rain). If you leave it on a cool table, it stays full. But if you put that same cup on a hot stove (rising temperatures), the water evaporates faster, even if you didn't pour any out. The cup looks emptier, not because you lost the water, but because the heat "stole" it.

The Two Different "Thirst Meters"

To understand this, the researchers used two different tools to measure drought:

The Rain Meter (SPI): This only counts the rain. It says, "Hey, we got the same amount of rain as before, so we are okay."
The Heat-Aware Meter (SPEI): This counts the rain minus the water lost to the heat. It says, "Wait, the heat is so strong now that even with the same amount of rain, the soil is actually much thirstier."

The Discovery: Since around 2003, these two meters have started to disagree. The "Rain Meter" says things are fine, but the "Heat-Aware Meter" screams that the region is getting drier. The heat is stealing the moisture before the plants can drink it.

The "Permanent Thirst"

The study found that this entire region is in a state of permanent thirst.

The Analogy: Think of the region as a person who is always slightly hungry. Even in the best years, they never feel fully full. The researchers calculated a "Moisture Score" for every single spot on the map, and every single spot was negative. This means the land is always asking for more water than it gets.

The "Sudden Heat Wave" Moment

The researchers found a specific moment in time when things changed. Around 2001–2003, the temperature jumped up like a switch was flipped.

Before 2003: The heat was rising slowly.
After 2003: The heat accelerated.
This sudden jump is why the "Heat-Aware Meter" started showing much worse drought conditions after 2003, even though the rain didn't change.

What This Means for Farmers and Water

The paper connects these findings to real-life struggles in Northern Togo:

The Crops: Farmers grow crops like corn and millet. Because the heat is stealing water, these crops are suffering. The study notes that in 2025, the conditions were so dry (according to the heat-aware meter) that crop yields could drop by 25% to 35%. It's like trying to bake a cake with the oven door open; the ingredients are there, but the heat ruins the result.
The Water Supply: The region relies on shallow wells and a big dam (the Kozah dam) for drinking water. Because the heat evaporates the water so fast, the dam levels dropped dramatically in 2025.
- The Real-World Impact: The water level in the dam fell by half. This forced the government to take emergency action, like digging new wells, because the main source of water was running dry. The study says this crisis happened because the heat is "sucking up" the rain before it can soak into the ground to refill the underground water tanks.

The Bottom Line

The main takeaway is that we cannot just look at the rain anymore.

If you only watch the rain gauge, you might think Northern Togo is doing fine. But if you look at the combination of rain and rising heat, you see a region that is becoming increasingly dry and vulnerable. The "invisible" drought caused by heat is just as dangerous as the "visible" drought caused by a lack of rain.

The study concludes that to save the crops and the drinking water, people need to prepare for a hotter, thirstier future, not just a drier one.

Technical Summary: Hydroclimatic Variability and Drought Characterisation in Northern Togo, 1981-2025

Problem Statement
Northern Togo, situated within the Sudanian savanna and sub-Sahelian transition zones, is highly vulnerable to rainfall variability and water scarcity, yet it lacks locally adapted tools for effective drought monitoring and moisture deficit assessment. While neighboring countries like Nigeria, Ghana, and Burkina Faso have seen extensive hydrological research, Togolese literature remains scarce. Existing studies in Togo often rely on sparse station networks without spatial extension via reanalysis or satellite products, limiting their representativeness. Furthermore, there is a critical gap in understanding how rising temperatures may be intensifying drought risks independently of precipitation trends (the SPI-SPEI divergence) and in quantifying the permanent water deficit regime across the region's three agro-ecological zones using the Willmott-Feddema Moisture Index (MI).

Methodology
This study analyzed hydroclimatic variability from 1981 to 2025 across three agro-ecological zones: Tall Grass Savanna (Sokodé), Short Grass Savanna (Kara), and Marginal Savanna (Mango). The research integrated:

Data Sources: Monthly precipitation and temperature data from three synoptic stations (1981–2025), NASA POWER reanalysis data (0.5° resolution, 1981–2024), and CHIRPS v2.0 satellite precipitation data (0.05° resolution, 1981–2024).
Trend Analysis: The Hamed-Rao modified Mann-Kendall test and Sen's slope estimator were applied to detect trends in annual, seasonal, and monthly timescales.
Change-Point Detection: Three non-parametric tests (Pettitt, Buishand, and Standard Normal Homogeneity Test) were used to identify structural breaks in climate series.
Drought Indices:
- SPI (Standardized Precipitation Index): Calculated at a 12-month scale to assess meteorological drought.
- SPEI (Standardized Precipitation-Evapotranspiration Index): Calculated using the water balance ( $D_i = P_i - PET_i$ ), where Potential Evapotranspiration (PET) was estimated via the Hargreaves-Samani method.
- MI (Willmott-Feddema Moisture Index): Used to quantify the annual water balance and permanent aridity.
Spatial Analysis: Spearman's rank correlation was employed to identify geographical covariates (latitude, altitude, PET) regulating moisture deficits across 84 NASA POWER grid points.

Key Results

Precipitation vs. Temperature Trends: No statistically significant trends in annual precipitation were detected at any station ( $p > 0.40$ ). However, a highly significant warming trend was observed across all stations, ranging from +0.27 to +0.34 °C per decade ( $p < 0.001$ ).
Change Points: While precipitation change points were not statistically significant, a coherent temperature change point was identified between 2001 and 2003. This shift coincides with the Atlantic Multidecadal Oscillation (AMO) transitioning to a positive phase.
SPI-SPEI Divergence: Since 2003, SPEI values have progressively diverged from SPI values, becoming more negative. This indicates that despite stable precipitation, increasing temperatures have amplified evapotranspiration demand, intensifying agricultural drought risk—a signal invisible to precipitation-only monitoring.
Permanent Water Deficit: All stations and grid cells recorded exclusively negative Moisture Index (MI) values throughout the study period, confirming a permanent water deficit across Northern Togo. MI values ranged from -2.9 (Kara) to -47.2 (Mango), with values consistently below the -20 semi-arid threshold.
Spatial Drivers: The north-south aridity gradient is primarily driven by latitude ( $\rho_s = -0.865$ ) and mean PET ( $\rho_s = -0.929$ ), rather than precipitation alone.
Recent Extremes: The study recorded severe drought events in 2025, with Mango reaching an extreme SPI of -2.16. This aligns with observed water supply crises, including a 50% drop in the Kozah dam water level (from 16m to 5.3m) by December 2025, reducing daily drinking water production by 40%.

Significance and Claims
The paper claims that the hydrological dynamics in Northern Togo are more complex than previously characterized. Its primary contribution is demonstrating that drought risk is increasing due to warming-driven evapotranspiration, even in the absence of declining precipitation trends. The study highlights that:

Monitoring Systems: Current precipitation-only monitoring systems fail to capture the intensification of agricultural drought caused by rising temperatures.
Water Security: The divergence between SPI and SPEI since 2003 signals a decrease in aquifer recharge efficiency, threatening both rain-fed cereal production (with potential yield losses of 25–35%) and domestic water supplies.
Policy Implications: The findings underscore the urgent need for temperature-sensitive drought indices in early warning systems and advocate for adaptation measures such as managed aquifer recharge, diversification of water sources, and the adoption of drought-tolerant crop varieties to support SDG 2 (Zero Hunger), SDG 6 (Clean Water), and SDG 13 (Climate Action).

The study concludes that while the region shows signs of partial Sahelian recovery in precipitation since 1990, the structural water deficit remains, exacerbated by the post-2003 warming trend.