An Analysis of West African Dynamics Using a Linearized GCM*

We used the Abdu Salam International Centre for Theoretical Physics (ICTP) Regional Climate Model version 4.5 (RegCM4.5), to investigate the potential impacts of land cover change of the Sahel–Sahara interface on the West African climate over an interannual timescale from 1990 to 2009. A simulation at 50 km grid spacing is performed with the standard version of the RegCM4.5 model (control run), followed by three vegetation change experiments at the Sahel-Sahara interface (15° N and 20° N): forest, tall grass, and short grass savanna. The impacts of land cover change are assessed by analyzing the difference between the altered runs and the control one in different sub-domains (western Sahel, central Sahel, eastern Sahel, and Guinea). Results show that the presence of forest, tall grass, and short grass savanna at the Sahel–Sahara interface tends to decrease the mean summer surface temperature in the whole domain. Nevertheless, this decrease is more pronounced over the Central Sahel when considering the forest experiment. This temperature decrease is associated with a weakening (strengthening) of the sensible (latent) heat flux in the whole domain. An analysis of the radiation field is performed to better explain the changes noted in the latent heat flux, the sensible heat flux, and the surface temperature. When considering the rainfall signal, the analysis shows that the afforestation options tend to alter the precipitation in the considered sub-domains substantially by increasing it in the whole Sahel region, with strong interannual variability. This rainfall increase is associated with an increase of the atmospheric moisture. Finally, we investigated the impacts of the afforestation options on some features of the rainfall events, and on the atmospheric dynamics during wet and dry years. All afforestation options tend to increase the frequency of the number of rainy days in regions located south of 18° N during both periods. Nevertheless, this increase is stronger over the Central and Eastern Sahel during wet years in the forest case. All afforestation experiments induce an increase (decrease) of the low-levels monsoon flux in the Eastern Sahel (western Sahel) during both periods. At the mid-levels, the three afforestation options tend to move northward and to decrease the intensity of the African Easterly Jet (AEJ) south of 13° N during wet and dry years.The intensity of the AEJ is weaker during the wet period. The vegetation change experiments also affect the Tropical Easterly Jet (TEJ), especially during wet years, by increasing its intensity over the southern Sahel. The analysis of the activity of African Easterly Waves (AEWs) patterns exhibits a decrease of the intensity of these disturbances over the Sahel during both periods. This may be due to the weakening of the meridional temperature contrast between the continent and the Gulf of Guinea due to the Sahel–Sahara surface temperature cooling induced by the afforestation. In summary, this study shows that during both periods, the increase of the atmospheric moisture due to the afforestation is associated with favorable AEJ/TEJ configurations (weaker and northward position of the AEJ; stronger TEJ) which in turn may create a stronger convection and then, an increase in the Sahel rainfall. This Sahel rainfall increase is associated with a strengthening of the intense and heavy rainfall events which may impact diversely local populations.

Hydrological models often perform poorly in simulating dry years in regions with large inter-annual variability in rainfall. We calibrated the Soil and Water Assessment Tool (SWAT) model to dry and wet years separately, using the semi-arid Barrett watershed on the west coast of USA as an example. We used hydrological and meteorological data from 1980–2010 to calibrate the SWAT model parameters, compared the monthly runoff results simulated by SWAT using a traditional calibration for the entire runoff series with results using a calibration with the wet and dry year series, and analyzed differences in the most sensitive parameters between the wet and dry year series. The results showed that (1) the SWAT model calibrated to the entire runoff series produced significant differences in simulation efficiency between the wet years and dry years, with lower efficiency during the dry years; (2) the calibration with separate wet and dry years greatly enhanced the SWAT model’s simulation efficiency for both wet and dry years; (3) differences in hydrological conditions between wet and dry years were represented by changes in the values of the six most sensitive parameters, including baseflow recession rates, channel infiltration rates, Soil Conservation Service (SCS) curve number, soil evaporation, shallow aquifer flow, and soil water holding capacity. Future work can attempt to determine the physical processes that underlie these parameter changes and their impact on the hydrological response of the semi-arid watersheds.

The relationship of δD and δ18O in soil water and its implications for soil evaporation across distinct rainfall years in winter wheat field in the North China Plain

The present study investigates the year-to-year variations of September–October rainfall in Hainan, China, for the period 1965–2010. The dominant circulation anomalies feature a cyclone (an anticyclone) over the Indochina Peninsula and northern South China Sea, an anticyclone (a cyclone) over subtropical western North Pacific and lower-level convergence (divergence) over the Maritime Continent in the wet (dry) years. These circulation anomalies are responses to an east–west sea surface temperature (SST) anomaly pattern with negative (positive) SST anomalies in the equatorial central Pacific and positive (negative) SST anomalies around the Maritime Continent in the wet (dry) years. Although the SST anomaly pattern is similar (but with opposite anomaly), the SST signal in the equatorial central Pacific is more significant in the dry years than in the wet years. This difference indicates a larger case-to-case variability in the wet years than in the dry years. The large variability in the wet years is attributed to contributions of tropical cyclones (TCs) and intraseasonal oscillations (ISOs). There are more TCs impinging on Hainan and the TC tracks are closer to the island in the wet years than in the dry years. The rainfall shows large intraseasonal variations with periods of 10–20 and 30–60 days during September–October in the wet years. The 10–20-day ISO originates from the Maritime Continent, whereas the 30–60-day ISO develops over tropical Indian Ocean and propagates northeastward to northern South China Sea. In contrast, the ISO signal is much weaker in the dry years.

Implementation of the Grain-for-Green project has led to rapid land cover changes and resulted in a significantly increased vegetation cover on the Loess Plateau of China during the past few decades. The main objective of this study was to examine the responses of soil water dynamics under four typical vegetation types against precipitation years. Soil water contents (SWCs) were measured in 0–4.0 m profiles on a hillslope under the four vegetation types of shrub, pasture, natural fallow and crop in a re-vegetated catchment area from April to October in normal (2010), dry (2011), wet (2014) and extremely wet (2013) years. The results indicated that precipitation and vegetation types jointly controlled the soil water temporal dynamics and profile characteristics in the study region. SWCs in 0–4.0 m profiles of the four vegetation types were ranked from high to low as crop>fallow>pasture>shrub and this pattern displayed a temporal stability over the four years. In the extremely wet year, SWC changes occurred in the 0–2.0 m layer under shrub and pasture while the changes further extended to the depth of 4.0-m deep layers under fallow and crop. In the other three years, SWCs changes mainly occurred in the 0–1.0 m layer and kept relatively stable in the layers deeper than 1.0 m for all the four vegetation types. The interannual variation in soil depth of SWCs was about 0–2.0 m for shrub and pasture, about 0–3.4 m for fallow and about 0–4.0 m for crop, respectively. The dried soil layers formed at the depths of 1.0, 0.6, 1.6 and 0.7 m under shrub, and 1.0, 1.0, 2.0 and 0.9 m under pasture, respectively in 2010, 2011, 2013 and 2014. The infiltrated rainwater mostly stayed in the 0–1.0 m layer and hardly supplied to soil depth >1.0 m in normal, dry and wet years. Even in the extremely wet year of 2013, rainwater recharge depth did not exceed 2.0 m under shrub and pasture. This implied that soil desiccation was difficult to remove in normal, dry and wet years, and soil desiccation could be removed in 1.0–2.0 m soil layers even in the extremely wet year under shrub and pasture. The results indicated that the natural fallow was the best vegetation type for achieving sustainable utilization of soil water and preventing soil desiccation.

This study examines selected dynamical factors associated with wet and dry years in the West African Sahel. The approach is to evaluate the temperature, wind, and moisture fields and the dynamic instabilities for a 4-yr“wet” composite (1958–61) and a 4-yr “dry” composite (1982–85). The analysis, limited to the months of June through September, is carried out using the National Centers for Environmental Prediction–National Center for Atmospheric Research (NCEP–NCAR) 40-Year Reanalysis Project dataset. Two upper-air datasets are also evaluated to confirm trends apparent in the NCEP–NCAR data. This study confirms some of the results of earlier studies, such as the weaker African easterly jet (AEJ) and stronger tropical easterly jet (TEJ) during wet years, but suggests a different interpretation of the wet–dry contrasts. In the Sahel, the most important characteristic of the AEJ appears to be its latitudinal location rather than its intensity. This governs the instability mechanisms. The AEJ is displ...

Interannual rainfall extremes over southwest Western Australia (SWWA) are examined using observations, reanalysis data, and a long-term natural integration of the global coupled climate system. The authors reveal a characteristic dipole pattern of Indian Ocean sea surface temperature (SST) anomalies during extreme rainfall years, remarkably consistent between the reanalysis fields and the coupled climate model but different from most previous definitions of SST dipoles in the region. In particular, the dipole exhibits peak amplitudes in the eastern Indian Ocean adjacent to the west coast of Australia. During dry years, anomalously cool waters appear in the tropical/subtropical eastern Indian Ocean, adjacent to a region of unusually warm water in the subtropics off SWWA. This dipole of anomalous SST seesaws in sign between dry and wet years and appears to occur in phase with a large-scale reorganization of winds over the tropical/subtropical Indian Ocean. The wind field alters SST via anomalous Ekman transport in the tropical Indian Ocean and via anomalous air–sea heat fluxes in the subtropics. The winds also change the large-scale advection of moisture onto the SWWA coast. At the basin scale, the anomalous wind field can be interpreted as an acceleration (deceleration) of the Indian Ocean climatological mean anticyclone during dry (wet) years. In addition, dry (wet) years see a strengthening (weakening) and coinciding southward (northward) shift of the subpolar westerlies, which results in a similar southward (northward) shift of the rain-bearing fronts associated with the subpolar front. A link is also noted between extreme rainfall years and the Indian Ocean Dipole (IOD). Namely, in some years the IOD acts to reinforce the eastern tropical pole of SST described above, and to strengthen wind anomalies along the northern flank of the Indian Ocean anticyclone. In this manner, both tropical and extratropical processes in the Indian Ocean generate SST and wind anomalies off SWWA, which lead to moisture transport and rainfall extremes in the region. An analysis of the seasonal evolution of the climate extremes reveals a progressive amplification of anomalies in SST and atmospheric circulation toward a wintertime maximum, coinciding with the season of highest SWWA rainfall. The anomalies in SST can appear as early as the summertime months, however, which may have important implications for predictability of SWWA rainfall extremes.

Carbon dioxide (CO2) efflux from soil (or soil respiration, SR) is one of the most important yet variable characteristics of soil. When evaluating large areas, CO2 efflux modeling serves as a viable alternative to direct measurements. This research aims to identify site-specific differences and their effects on empirical CO2 efflux modeling. The experimental data from 25 years of field observations were utilized to identify the optimal site- and weather-specific models, parameterized for normal, wet, and dry years, for the forest and grassland ecosystems located on similar Entic Podzols (Arenic) in the same bioclimatic coniferous–deciduous forest zone. The following parameters were considered in the examined models: mean monthly soil or air temperatures (Tsoil and Tair), amount of precipitation during the current (P) and the previous (PP) months, and the storage of soil organic carbon (SOC) in the top 20 cm of soil. The weighted non-linear regression method was employed to estimate the model parameters for the normal, wet, and dry years. To increase the magnitude of the model resolutions, we controlled the slope and intercept of the linear model comparison between the measured and modeled data through the change in R0—CO2 efflux at Tsoil = 0 °C. The mean bias error (MBE), root-mean-square error (RMSE), and determination coefficient (R2) were employed to assess the quality of the model’s performance. The measured Tsoil, Tair, and P, as well as the litter (for forest) or sod (for grassland) horizon (modeled by the Soil SCLmate Statistical Simulator (SCLISS)), and soil temperatures (Tlit_m, Tsoil_m) and moistures (Mlit_m, Msoil_m), were used for SR simulation. For the CO2 efflux in the forest ecosystem with the lower SOC availability for mineralization, the direct Tsoil and Tair measurements in combination with SOC storage provided better parameterization for the empirical TPPC model. For the CO2 efflux in the grassland ecosystem with the high SOC availability for mineralization, the temperature became the governing factor, and the TPPrh model provided better performance over all the considered models. The model’s performance was the best for the wet years, and the worst for the dry years for both ecosystems. For forest ecosystems, the model performance for average precipitation years was equivalent to that in wet years. For grassland ecosystems, however, the model performance was equivalent to that in dry years due to differing exposure and hydrothermal regimes. The wet-year R0 obtained for both forest and grassland ecosystems differed from the normal- and dry-year values. The measured SR values relevant for the R0 estimations distribute along the precipitation range for the forest and along the temperature range for the grassland. The SCLISS-modeled Tlit_m and Mlit_m provide good alternatives to direct atmospheric measurements, and can be used as initial temperature and moisture data for CO2 efflux modeling when direct soil and moisture observations are not available on site.

Saguling Reservoir has a potential to be used as a raw water supply for Bandung Metropolitan Area (BMA), with the discharge of 1.622 dm·s−1. However, further studies are needed to ensure that the water quality is in accordance with the government regulations. This study shows that the reservoir’s P concentration was 315.0 mg·m−3 on average in 1999–2013. This value only meets the class III of government standards of water quality for the cultivation of freshwater fish, livestock, and to irrigate landscaping, but does not belong to the class I standard of 200 mg·m−3 for drinking water. The total-P concentration in wet, normal, and dry years was 796.3, 643.8, and 674.8 mg·m−3, respectively. The pollution load was highest in wet years due to the high levels of sediment. The pollution load of the reservoir did not exceed the class III classification of 29 405.01 kg·year−1. The pollution load in wet, normal, and dry years was 38 790.1, 25 991.9, and 23 929.0 kg·year−1, respectively. The phosphorus pollution is caused by the use of floating net cages in the reservoir, which makes it difficult to meet the standards. In wet years, the pollution load was higher than in normal and dry years. The P load could be higher in the wet season due to dilution and could probably decrease the pollutant concentration in the reservoir.

Water vapor characteristics over northeast Brazil for two contrasting years are discussed. During the wet year 1974, the precipitation efficiencies were less than 20% over the interior dry region and during the dry year 1976, they were reduced to 10% or less. The reduction is mainly due to low precipitation in 1976. Calculation of water vapor flux showed that in both wet and dry years, the flux is inward in the east sector and outward in the west sector. This suggests the importance of the South Atlantic Ocean in providing water vapor for NE Brazil. During the wet year, the lower tropospheric flow is convergent and in the northwest part, the direction of flow is more northerly/less southerly. During the dry year the opposite occurred. This indicates the possible role of the Intertropical Convergence Zone in the interannual variations of climate over NE Brazil.

In recent years, Germany has experienced an increasing number of extreme wet and dry years. In the North German lowlands, wet grassland sites with shallow water table conditions are widespread landscape elements. They are characterized by a special water and nutrient balance that reacts very sensitively to changes in the hydrological system. Studies on evapotranspiration (ETa) and the development of groundwater levels were carried out at two typical wet grassland sites with shallow water table conditions. A weighable groundwater lysimeter system in the Spreewald wetland (SPW) and an eddy covariance station in Havelländisches Luch (HL) were used to measure ETa. The results show that even these shallow water table sites cannot sufficiently meet the vegetation’s water demands in extreme dry conditions. The groundwater levels drop to values deeper than 1 m below the surface. As a result, water supply to the vegetation is temporarily limited. The mean crop coefficients (Kc) of these wet grassland sites reach values of 1.1 in the vegetation period with a sufficient water supply, but drop to around 0.8 in dry years when the water supply is limited. Areas with small catchment areas, such as HL, are more seriously affected by the dry meteorological conditions than areas with sufficient inflows from larger catchment areas, such as SPW.

Research was conducted to determine the effects of feedlot surfacing materials (soil/unpaved vs. fly-ash paved), partial composting and/or storage in windrow or greenhouse, and wet year (2005) vs. dry year (2006) feedlot conditions on cattle feedlot biomass (FB) characteristics pertinent to combustion or gasification processes involving reburn or co-firing with coal or lignite as described elsewhere. Bulk quantities of FB were harvested from 12 fly-ash paved pens and six soil-surfaced pens in a 400-head research cattle feedlot near Bushland, Texas. FB was windrow-composted, and samples were analyzed using proximate, ultimate, and elemental ash analyses. Higher heating value (HHV) before composting (termed raw manure, RM) was significantly higher for FB harvested from paved pens (termed low ash, or LA-FB) vs. soil-surfaced pens (termed high ash, or HA-FB) on a dry basis and dry ash-free (DAF) basis for both study years. Ash content (dry matter basis) was significantly lower and volatile matter (VM), carbon (fixed and total), and hydrogen were significantly higher for LA-FB (paved pens) vs. HA-FB (unpaved pens) for both years of the study. As-harvested HHV averaged 6,304 and 8,190 kJ kg-1 w.b. (2,710 and 3,521 BTU lb-1 w.b.) for HA-FB for the wet and dry years, respectively, and as-harvested HHV for LA-FB averaged 13,407 and 13,224 kJ kg-1 w.b. (5,764 and 5,685 BTU lb-1 w.b.) for the wet and dry years. Partial composting (PC) in windrows (for 51 to 55 days in 2005 or 91 days in 2006) reduced HHV w.b. by an average (both years) of 11.4% for the resulting HA-FB-PC and 14.8% for the resulting LA-FB-PC. The LA-FB-PC was significantly different from HA-FB-PC for all proximate and ultimate analysis parameters for both 2005 and 2006 manure, with the exception of 2006 wet-basis HHV. Heating value on a DAF basis averaged 20,984 kJ kg-1 (9,022 BTU lb-1) for LA-FB-PC and 18,941 kJ kg-1 (8,143 BTU lb-1) for HA-FB-PC. Greenhouse storage for 9 to 12 months of the 2005 harvested LA-FB-RM and LA-FB-PC preserved fuel quality better than did continuous storage of LA-FB-PC in a windrow, probably because lower moisture conditions were maintained under greenhouse storage conditions (7.7% to 13.4% w.b. final range) compared with uncovered windrow storage (17.4% final moisture content). Elemental analysis of ash from LA-FB was higher than from HA-FB for Ca, P, Cl, K, Mg, Na, and S, but was lower for Si, Al, Ti, and Fe without or with partial composting. Metal contents were similar for both high-ash and low-ash FB.

Roughly one-third of the Earth's land surface is seasonally covered by snow. In many of these ecosystems, the spring snowpack is melting earlier due to climatic warming and atmospheric dust deposition, which could greatly modify soil water resources during the growing season. Though snowmelt timing is known to influence soil water availability during summer, there is little known about the depth of the effects and how long the effects persist. We therefore manipulated the timing of seasonal snowmelt in a high-elevation mixed-conifer forest in a Mediterranean climate during consecutive wet and dry years. The snow-all-gone (SAG) date was advanced by 6 days in the wet year and 3 days in the dry year using black sand to reduce the snow surface albedo. To maximize variation in snowmelt timing, we also postponed the SAG date by 8 days in the wet year and 16 days in the dry year using white fabric to shade the snowpack from solar radiation. We found that deeper soil water (30–60 cm) did not show a statistically significant response to snowmelt timing. Shallow soil water (0–30 cm), however, responded strongly to snowmelt timing. The drying effect of accelerated snowmelt lasted 2 months in the 0–15 cm depth and at least 4 months in the 15–30 cm depth. Therefore, the legacy of snowmelt timing on soil moisture can persist through dry periods, and continued earlier snowmelt due to climatic warming and windblown dust could reduce near-surface water storage and availability to plants and soil biota.

Evident response of future hydropower generation to climate change

Studies of the ecological regimes of groundwater and moisture reserves were carried out in the Novgorod Region in 2014–2023 in conditions of arid (HTC 0.9...1.1) and wet vegetation periods (HTC 1.69...2.46) on sod-podzolic loamy soils drained by closed drainage structures: shallow with a depth of 70 cm; two variants of a medium-deep drainage with a depth of 110 cm with filling of the drainage trench with wood chips and sand-gravel mixture; two-tiered, consisting of a lower tier of tubular drains with a depth of 110 cm and an upper tier of tubeless drains perpendicular to it, located at a depth of 60 cm. Analysis of the nature of changes in the groundwater level showed that the minimum depth of groundwater in both wet and dry years was observed at the beginning of the growing season, the second peak of groundwater rise in wet years was observed in August, in dry years it was absent. The highest average seasonal groundwater level was observed in excessively humid 2017 (HTC = 2.46) in the variants of medium–deep drainage with backfilling of chips (30 cm) and shallow drainage (33 cm), the lowest – in the variant of two-tier drainage (58 cm). Analysis of humidity regimes on experimental systems showed that with shallow drainage in a humid year, more than a third, and in an excessively humid year, more than 2/3 of the duration of the growing season, the upper 30-centimeter horizon was in a waterlogged state, and with two-tier drainage, there were no periods of excessive moisture. At the same time, in wet years, the probability of periods with insufficient soil moisture ranged from 8 % (with medium-deep drainage with sand-gravel mixture) to 25 % (with two-tier drainage) In dry years, there was a lack of moisture in the upper soil horizon in the two-tier drainage variant of about 70 %, in other variants this value was slightly lower (62–65 %). The ecological soil regimes formed under the influence of two-tier drainage in modern climatic conditions had a significant impact on the hay yield of perennial grasses (70 % of fodder cereals), which allowed for an average yield increase of 0.9 t/ha relative to the variant with shallow drainage during both wet and dry growing seasons.