Tag: precipitation
Water table gradient effects on the performance of diverse cowpea cultivars
Cowpea sown in tropical monsoon climates is often constrained by soil saturation and a high water table during growth. We hypothesized that a strong interaction exists between cowpea maturity class and the soil water table depth regime experienced during the growing season, and that the occurrence of shallow water table depths in the dry-wet transition period before rice and the rapid decline in water table depth in the wet-dry post-rice period favor early maturing cultlvars. The effects of three water table regimes on the performance of 24 diverse cowpea [Vigna unguiculat (L.) Walp] cultivars was studied on a Typic Tropudalf toposequence. The water table depths in the shallow (SWT), medium (MWT), and deep (DWT) water table regimes varied between 0.06 to 0.54 m, 0.28 to 0.96 m, and 0.68 to 1.44 m in the 2 yr of dry season (DS) experiments. In the wet season fWS), the three regimes were 0 to 0.27 m, 0.30 to 0.60 m, and 0.55 to 1.05 m. Most of the early maturing cultivars exhibited their best performance in the DWT regime, but the medium maturing cultivars were superior in seed yields in all water table regimes. The mean yields of the early maturing cultivars were reduced by 49% (1986–1987 DS) and 57% (1987–1988 DS) in SWT compared to DWT. They were reduced by 50% in the 1987 wet season. The mean yield reductions for the medium maturing cultivars were 40%, 54%, and 51%, respectively, in SWT compared to DWT. Pods per plant was the yield component most affected by excess moisture. Flowering in the SWT was delayed by 4 vs 5 d (DS) and 5 vs 7 d (WS) between the early and medium maturing cultivar groups, respectively. The superiority of the medium maturing cultivats was consistent among sites with a water table within the root zone for a major portion of the crop season.
Improving the Efficiency of Runoff Pond System for Supplementary Irrigation in Arid and Semi-arid Areas of Kenya
With the advent of climate change, semi-arid regions are witnessing increased variability of weather patterns depicted in changes of amount and onset of precipitation, high evapotranspiration demands and increased frequencies of famines. This has exacerbated food security situation, culminating in increased demand for irrigation to mitigate against dry spells and drought. In the semi-arid regions of Eastern Kenya, most farmers are adopting the harnessing of runoff ponds to create water buffer that would be used during the crucial crop-growing stages. Thus, a runoff pond system is comprised of conveyance, storage, abstraction and application mechanisms. However, the efficiency of the system along each component is still low, owing to water losses through poor transmission, seepage, leakage and evaporation. This chapter highlights on experiences of farmers, in Kibwezi East sub-county, Masongaleni location, who had installed 140 ponds by the end of 2013 with new ones still being dug. It goes further to recommend on best practices that could help improve the systems’ performance of these runoff ponds, and how lessons learnt from here could help improve similar initiatives in the eastern and southern sub-Saharan Africa.
Ground- and satellite-based evidence of the biophysical mechanisms behind the greening Sahel
After a dry period with prolonged droughts in the 1970s and 1980s, recent scientific outcome suggests that the decades of abnormally dry conditions in the Sahel have been reversed by positive anomalies in rainfall. Various remote sensing studies observed a positive trend in vegetation greenness over the last decades which is known as the re-greening of the Sahel. However, little investment has been made in including long-term ground-based data collections to evaluate and better understand the biophysical mechanisms behind these findings. Thus, deductions on a possible increment in biomass remain speculative. Our aim is to bridge these gaps and give specifics on the biophysical background factors of the re-greening Sahel. Therefore, a trend analysis was applied on long time series (1987–2013) of satellite-based vegetation and rainfall data, as well as on ground-observations of leaf biomass of woody species, herb biomass, and woody species abundance in different ecosystems located in the Sahel zone of Senegal. We found that the positive trend observed in satellite vegetation time series (+36%) is caused by an increment of in situ measured biomass (+34%), which is highly controlled by precipitation (+40%). Whereas herb biomass shows large inter-annual fluctuations rather than a clear trend, leaf biomass of woody species has doubled within 27 years (+103%). This increase in woody biomass did not reflect on biodiversity with 11 of 16 woody species declining in abundance over the period. We conclude that the observed greening in the Senegalese Sahel is primarily related to an increasing tree cover that caused satellite-driven vegetation indices to increase with rainfall reversal
Critical climate periods for grassland productivity on China’s Loess Plateau
Strong correlations between aboveground net primary productivity (ANPP) of grasslands and mean annual temperature or precipitation have been widely reported across regional or continental scales; however, inter-annual variation in these climate factors correlates poorly with site-specific ANPP. We hypothesize that the reason for these weak correlations is that the impacts of climatic variation on grassland productivity depend on the timing and intensity of variation in temperature and precipitation. In this study, long-term records of grassland productivity on the Loess Plateau in China were related with daily temperature and precipitation during 1992–2011 using Partial Least Squares (PLS) regression to test the above-mentioned hypothesis. Our results suggested that temperature increases during the early stage of the growing season (April–May) were positively correlated with ANPP. However, these effects were canceled out when this phase was followed by a hot and dry summer (June–July). Impacts of drought and heat in August on productivity were negligible. Increased temperature and precipitation during the senescence period (September–October) and a warmer dormancy phase (November–March) were negatively correlated with productivity in the following year, while precipitation during the dormancy period had no detectable effects. Climatic variability in summer has thus far been the dominant driver of temporal variation in grassland productivity. Warming during winter and spring currently play minor roles, but it seems likely that the importance of these secondary impacts may increase as warming trends continue. This evaluation of climate variability impacts on ecosystem function (e.g. grassland productivity) implies that not only the magnitude but also the timing of changes in temperature and precipitation determines how the impacts of climate changes on ecosystems will unfold. © 2016 Elsevier B.V.
Who or what makes rainfall? Relational and instrumental paradigms for human impacts on atmospheric water cycling
Human impacts on water cycles (HIWC) can include modification of rainfall. Spatial and temporal variation in rainfall, with implications for ‘water security’, has been attributed to multiple causal pathways, with different options for human agency. Ten historical paradigms of the cause of rainfall imply shifts from ‘nature controlling humans’ to ‘human control over nature’ and ‘human control over other humans’. Paradigm shifts have consequences for human efforts, interacting with social–ecological systems, to appease spirits, please rainmakers, expose ‘rainfakers’, protect forest, plant trees, reduce greenhouse gas emissions, apply cloud seeding, or declare rainfall modification an illegitimate tool in warfare. The ‘instrumental’ and ‘relational’ values of atmospheric water cycling depend on cognitive paradigms of rainfall causation as represented in local, public/policy, or science-based ecological knowledge. The paradigms suggest a wide range of human decision points that require reinterpretation of rationality for any paradigm shift, as happened with the forest–rainfall linkages.
Ground-based climate data show evidence of warming and intensification of the seasonal rainfall cycle during the 1960–2020 period in Yangambi, central Congo Basin
Meteorological stations are rare in central Africa, which leads to uncertainty in regional climatic trends. This is particularly problematic for the Congo Basin, where station coverage decreased significantly during the last few decades. Here, we present a digitized dataset of daily temperature and precipitation from the Yangambi biosphere reserve, covering the period 1960–2020 (61 years) and located in the heart of the Congo Basin. Our results confirm a long-term increase in temperature and temperature extremes since the 1960s, with strong upward trends since the early 1990s. Our results also indicate a drying trend for the dry season and intensification of the wet season since the early 2000s. Ongoing warming and increasing precipitation seasonality and intensity already have a significant impact on crop yields in Yangambi. This calls for urgent development of climate-smart and dynamic agriculture and agroforestry systems. We conclude that systematic digitization and climate recording in the Congo Basin will be critical to improve much-needed gridded benchmark datasets of climatic variables.
Precipitation gradients drive high tree species turnover in the woodlands of eastern and southern Africa
Savannas cover one-fifth of the Earth’s surface, harbour substantial biodiversity, and provide a broad range of ecosystem services to hundreds of millions of people. The community composition of trees in tropical moist forests varies with climate, but whether the same processes structure communities in disturbance-driven savannas remains relatively unknown. We investigate how biodiversity is structured over large environmental and disturbance gradients in woodlands of eastern and southern Africa. We use tree inventory data from the Socio-Ecological Observatory for Studying African Woodlands (SEOSAW) network, covering 755 ha in a total of 6780 plots across nine countries of eastern and southern Africa, to investigate how alpha, beta, and phylogenetic diversity varies across environmental and disturbance gradients. We find strong climate-richness patterns, with precipitation playing a primary role in determining patterns of tree richness and high turnover across these savannas. Savannas with greater rainfall contain more tree species, suggesting that low water availability places distributional limits on species, creating the observed climate-richness patterns. Both fire and herbivory have minimal effects on tree diversity, despite their role in determining savanna distribution and structure. High turnover of tree species, genera, and families is similar to turnover in seasonally dry tropical forests of the Americas, suggesting this is a feature of semiarid tree floras. The greater richness and phylogenetic diversity of wetter plots shows that broad-scale ecological patterns apply to disturbance-driven savanna systems. High taxonomic turnover suggests that savannas from across the regional rainfall gradient should be protected if we are to maximise the conservation of unique tree communities.
The role of ecosystem transpiration in creating alternate moisture regimes by influencing atmospheric moisture convergence
The terrestrial water cycle links the soil and atmosphere moisture reservoirs through four fluxes: precipitation, evaporation, runoff, and atmospheric moisture convergence (net import of water vapor to balance runoff). Each of these processes is essential for human and ecosystem well-being. Predicting how the water cycle responds to changes in vegetation cover remains a challenge. Recently, changes in plant transpiration across the Amazon basin were shown to be associated exponentially with changes in rainfall, suggesting that even small declines in transpiration (e.g. from deforestation) would lead to much larger declines in rainfall. Here, constraining these findings by the law of mass conservation, we show that in a sufficiently wet atmosphere, forest transpiration can control atmospheric moisture convergence such that increased transpiration enhances atmospheric moisture import and resulting water yield. Conversely, in a sufficiently dry atmosphere increased transpiration reduces atmospheric moisture convergence and water yield. This previously unrecognized dichotomy explains the otherwise mixed observations of how water yield responds to re-greening, as we illustrate with examples from China’s Loess Plateau. Our analysis indicates that any additional moisture recycling due to additional vegetation increases precipitation but decreases local water yield and steady-state runoff. Therefore, in the drier regions/periods and early stages of ecological restoration, the role of vegetation can be confined to moisture recycling, while once a wetter stage is achieved, additional vegetation enhances atmospheric moisture convergence. Evaluating the transition between regimes, and recognizing the potential of vegetation for enhancing moisture convergence, are crucial for characterizing the consequences of deforestation as well as for motivating and guiding ecological restoration.
How transpiration by forests and other vegetation determines alternate moisture regimes
The terrestrial water cycle links the soil and atmosphere moisture reservoirs through four fluxes: precipitation, evaporation, runoff and atmospheric moisture convergence. Each of these fluxes is essential for human and ecosystem well-being. However, predicting how the water cycle responds to changes in vegetation cover, remains a challenge (Lawrence and Vandecar, 2015; Ellison et al., 2017; te Wierik et al., 2021). Recently, rainfall was shown to decrease disproportionally with reduced forest transpiration following deforestation (Baudena et al., 2021). Here, combining these findings with the law of matter conservation, we show that in a sufficiently wet atmosphere forest transpiration can control atmospheric moisture convergence such that increased transpiration enhances atmospheric moisture import. Conversely, in a drier atmosphere increased transpiration reduces atmospheric moisture convergence and runoff. This previously unrecognized dichotomy can explain the seemingly random observations of runoff and soil moisture sometimes increasing and sometimes reducing in response to re-greening (e.g., Zheng et al., 2021). Evaluating the transition between the two regimes is crucial both for characterizing the risk posed by deforestation as well as for motivating and guiding global ecosystem restoration.