Recent shifts in climate conditions have disrupted rainfall patterns in the Sudanian region of West Africa, causing local farmers to struggle with adjusting their farming strategies (Biasutti, 2019; Gbangou et al., 2019; Tzachor et al., 2023). This has led to the phenomenon known as the 'false onset' or 'false start' of the rainy season, which is characterized by erratic rain events at the beginning of the season, followed by intermittent dry spells of various lengths (Laux et al., 2008; Silungwe et al., 2019). Given that farming in the region is predominantly rainfed, and seasonal rainfall patterns largely guide planting decisions, traditional sowing strategies need to be reconsidered given these changing conditions. This research aims, therefore, to analyze the impact of changes in the dynamics of rainfall patterns in West Africa on crop yield for rainfed agriculture. Specifically, it will examine the effect of several different sowing strategies on crop yield using a crop simulation model.

First, it provides an overview of local farming practices in the Sudanian region of West Africa, alongside farmers' perspectives on climate change and the adaptive measures they use. The findings are based on interviews with local farmers in the region and discussions with research institutes and regional agencies involved in climate change, adaptation, and agricultural water management. Two main approaches were used during the interviews: semi-structured individual interviews based on a questionnaire and group discussions. The investigation revealed that farmers are acutely aware of the changing climate and its negative impact on crop production. The traditional practice of sowing after the first two rainfalls has become increasingly unreliable under current climate conditions. There is a consensus among farmers that the rainfall total has decreased, the duration of the season is shortening, and there is a high frequency of dry spells throughout the rainy season. Additionally, there is a notable lack of climate-based services to support farmers' decision-making processes. Consequently, smallholder farmers expressed a strong interest in receiving not only weather forecasts but also climate risk information, which is crucial for making agriculture in the region more resilient to climate change.

Chapter 3 examines various sowing strategies used in the rainfed farming system of West Africa, where intermittent dry spells during the rainy season exacerbate challenges and affect crop yield response. In the context of a changing climate, we hypothesize that the traditional focus on the onset of the season to start sowing leads to crop losses in years of high rainfall intermittency. Using AquaCrop, an FAO crop model, we simulate the crop response to varying water availability to sowing dates ranging from the 1st of May to the 30th of November at 20 locations in WA. The crop we focus on is maize (Zea mays L.). The analysis revealed that sowing directly after the first rains carries a higher risk of yield loss due to insufficient buildup of soil water storage to overcome dry spells. Based on three years of data per station on average between 2016 and 2020, various sowing strategies were evaluated. We identified safe sowing windows across the Sudanian region that secure optimal yield in 97% of all cases. We find that delaying sowing to mid-June (savanna and western part of the region) and to July (semi-arid region) ensures optimal yields. Of the three commonly applied local onset approaches covered in our evaluation, only the local onset LO10mm, defined as four consecutive days with 10 mm/day, achieves a similar yield result. The advantage of the safe sowing window approach is that it is easily accessible for smallholder farmers, who in many cases do not have access to local rainfall information.

Chapter 4 focuses on identifying sowing strategies that minimize yield losses at the regional level. Climate variability poses great challenges to farmers in the region who are heavily dependent on rainfall for farming. Hence, we investigate three sowing strategies available in the Sudanian region of West Africa (WA) to assess their ability to ensure crop yields for smallholder farmers under a changing climate. AquaCrop-GIS, the GIS version of the FAO crop model, is used to simulate the yield response of maize (Zea mays L.) to varying sowing dates (1st of May to 30th of November) throughout the rainy season across WA. Based on an average of 38 years of data per grid cell from the Global Precipitation Climatology Centre (GPCC), we identify safe sowing windows across the Sudanian region that secure at least 90% of maximal yield. The analysis revealed that the current sowing strategies, based on minimum thresholds for rainfall accumulated over a period, widely applied in the region, carry a higher risk of yield failure than the safe sowing window strategy, especially at the beginning of the rainy season. It also appears that delaying sowing for a month to mid-June in the central region (east of Lon 8.5°W), and to early August in the semi-arid areas is a safe strategy for smallholder farmers that ensures optimal yields. A long-term comparison between the periods 1982-1991 and 1992-2019 reveals earlier sowing dates for both LO10mm and LO20mm, indicating a shift towards a wetter regime relative to the reference period. In contrast, the safe sowing window strategy shows later sowing dates, underscoring the need for caution due to erratic early-season rainfall and variability beyond the control period defined by local sowing practices. The precipitation-based strategies hold a higher risk of yield failure compared to the safe sowing window strategy, which is more accessible to smallholder farmers and better suited to the current climate conditions.

The key findings of this dissertation contribute to regional efforts in building a climate-resilient agricultural system by equipping smallholder farmers with simple yet effective sowing strategies. The research recommendations offer practical, easily implementable solutions for smallholder farmers across West Africa. Given the dynamic nature of rainfall distribution and the uncertainty in future climate projections for West Africa, it is essential to strengthen regional initiatives that improve access to rainfall data and climate services. These efforts will equip farmers with the tools and knowledge needed to make informed decisions. As our understanding of rainfall patterns evolves, updating these recommendations will be crucial to mitigating the negative effects of dry spells and enhancing crop productivity across the region. Additionally, regional efforts should prioritize enhancing and expanding climate and weather monitoring infrastructure to improve our understanding of the region’s complex weather systems at a finer scale. This will enable more precise modeling of the intricate weather-crop-soil interactions and provide localized information to smallholder farmers. Leveraging crop simulation models will facilitate the exploration of both current and future scenarios, helping to identify the most efficient farming practices. This will ultimately improve water use efficiency and contribute to closing the food security gap.

Climate variability poses great challenges to food security in West Africa, a region heavily dependent on rainfall for farming. Identifying sowing strategies that minimize yield losses for farmers in the region is crucial to securing their livelihood. In this paper, we investigate three sowing strategies to assess their ability to identify safe sowing windows for smallholder farmers in the Sudanian region of West Africa (WA) in the context of a changing climate. The GIS version of the FAO crop model, AquaCrop-GIS, is used to simulate the yield response of maize (Zea mays L.) to varying sowing dates throughout the rainy season across WA. Based on an average of 38 years of data per grid cell, we identify safe sowing windows across the Sudanian region that secure at least 90% of maximal yield. We find that current sowing strategies, based on minimum thresholds for rainfall accumulated over a period that are widely applied in the region, carry a higher risk of yield failure, especially at the beginning of the rainy season. This analysis shows that delaying sowing for a month to mid-June in the central region (east of Lon 8.5°W), and to early August in the semi-arid areas is a safer strategy that ensures optimal yields. A comparison between the periods 1982–1991 and 1992–2019 shows a negative shift for LO10 mm and LO20 mm, suggesting a wetter regime compared to the dry periods of the 1970s and 1980s. On the contrary, we observe a positive shift in the safe window strategy, highlighting the need for precautions due to erratic rainfall at the beginning of the season. The precipitation-based strategies hold a high risk, while the safe sowing window strategy, easily accessible to smallholder farmers, is more fitting, given the current climate.

Flash droughts, characterized by rapid onset and intensification, are increasingly occurring as a consequence of climate change and rising temperatures. However, existing hydrometeorological definitions fail to encompass the full range of flash droughts, many of which have distinct local physical attributes. Consequently, these events often go undetected or unforecast in generic global flash drought assessments and are underrepresented in research. To address this gap, we conducted a comprehensive survey to gather information on local nomenclature, characteristics, and impacts of flash droughts worldwide. The survey revealed the widespread occurrence of these phenomena, highlighting their underre-searched nature. By analyzing case studies, through literature review often in local languages to unearth elusive studies, we identified five different types of flash droughts based on their specific characteristics. Our study aims to increase awareness about the complexity and diverse impacts of flash droughts, emphasizing the importance of considering regional contexts and the vulnerability of affected populations. The reported impacts underscore the need for better integration of all flash drought types in drought research, monitoring, and management. Monitoring a combination of indicators is crucial for timely detection and response to this emerging and escalating threat.

Climate change is exacerbating adverse impacts of water stress in rainfed agriculture. This paper seeks to identify safe sowing windows for smallholder farmers in the Sudanian region of West Africa (WA). We hypothesize that the traditional focus on the onset of the season to start sowing leads to crop losses in years of high rainfall intermittency. AquaCrop, an FAO crop model, is used to simulate the yield response of maize (Zea mays L.) to sowing dates ranging from the 1st of May to the 30th of November at 20 locations in WA. We find that sowing directly after the first rains carries a higher risk of water stress, hampering crop development due to insufficient buildup of soil water storage to overcome dry spells. Based on three years of data per station on average, we identify safe sowing windows across the Sudanian region that secure optimal yield in 97% of all cases. We find that delaying sowing to mid-June (savanna and western part of the region) and to July (semi-arid region) ensures optimal yields. Of the three commonly applied local onset approaches covered in our evaluation, only LO10mm (10 mm/day on four consecutive days) achieves a similar yield result. The advantage of the safe window approach is that it is accessible for smallholders, who in many cases do not have access to local rainfall information.

This paper shares an early-career perspective on potential themes for the upcoming International Association of Hydrological Sciences (IAHS) Scientific Decade (SD). This opinion paper synthesizes six discussion sessions in western Europe identifying three themes that all offer a different perspective on the hydrological threats the world faces and could serve to direct the broader hydrological community: “Tipping points and thresholds in hydrology,” “Intensification of the water cycle,” and “Water services under pressure.” Additionally, four trends were distinguished concerning the way in which hydrological research is conducted: big data, bridging science and practice, open science, and inter- and multidisciplinarity. These themes and trends will provide valuable input for future discussions on the theme for the next IAHS SD. We encourage other early-career scientists to voice their opinion by organizing their own discussion sessions and commenting on this paper to make this initiative grow from a regional initiative to a global movement.