Negative Water Budget Research Articles

The African Holocene Humid Period in the Tibesti mountains (central Sahara, Chad): Climate reconstruction inferred from fossil diatoms and their oxygen isotope composition

The climate of the African Holocene Humid Period (AHHP) is reconstructed in the Tibesti Volcanic Massif (TVM) in the central Sahara from well-preserved diatomaceous deposits in the two crater palaeolakes of Trou au Natron at Pic Toussidé and Era Kohor at Emi Koussi. The two records cover the period from ∼9500 to 4500 cal yr BP. Climate and palaeoenvironmental changes during this period were inferred from diatom assemblages, interpretation of variations in their oxygen isotope composition (δ18Odiatom), reconstruction of lake water conductivity from diatom-based transfer functions, and estimation of the lake water balance (Evaporation/Inflow ratio, E/I). Our findings provide evidence for two distinct lacustrine episodes. During the early to mid-Holocene transition, low δ18Odiatom values, high percentages of planktonic diatoms, low lake water conductivity and a positive water balance (E/I < 1) suggest wet conditions, which were likely related to the optimum of the AHHP. From the mid-to late Holocene transition, an aridification trend is revealed by increasing δ18Odiatom values, high percentages of benthic diatoms and a negative water budget (E/I > 1), occurring as early as 6500 cal yr BP and intensifying after 5300 cal yr BP. Moreover, our data show on average a decrease in precipitation amounts of ∼35% between the peak and the end of the AHHP in the Tibesti region. This timing of the AHHP in the mountainous Tibesti is consistent with the aridification of the central Sahara recorded at lowland sites, which has mainly been related to the southward retreat of the Intertropical Convergence Zone (ITCZ) and the associated African monsoonal rainfall belt, following the gradually declining summer insolation that led to the termination of the AHHP. Our results prove the existence of Holocene lakes in the TVM craters that developed contemporaneously with the lakes of the Chadian basin and the Libyan Sahara. On a broader scale, our data share similar hydroclimatic patterns with studies from the eastern and northern Sahara.

Hypersalinity accompanies tectonic restriction in the eastern Mediterranean prior to the Messinian Salinity Crisis

This study describes the hydroclimate evolution of the eastern Mediterranean Basin during the early Messinian (7.2 to 6.5 Ma) time-interval based on analysis of a succession at Agios Myron (Crete, Greece), prior to the onset of the Messinian Salinity Crisis (5.96–5.33 Ma). Specifically, we report sea surface temperature and salinity reconstructions based on a combined analysis of biomarkers and oxygen isotopes of planktonic foraminifera. Data reveal that a negative water budget and strong hydrologic and climate variability characterized the eastern Mediterranean Basin at this time, and we identify three distinct phases. In Phase 1 (7.2–6.9 Ma), a shift to more positive oxygen isotope values in planktonic foraminifera at ~7.2 Ma is attributed to progressive gateway restriction of Mediterranean–Atlantic corridors and subsequent cooling until 6.9 Ma. In Phase 2 (6.9–6.7 Ma), distinct warm and hypersaline events (at 6.9–6.82 and 6.72 Ma) resulted in stressed marine microfauna during periods of strong evaporation. An important step-change in the Mediterranean restriction at 6.72 Ma may have resulted from shallowing of the Mediterranean gateways and reduced Mediterranean marine outflow. During Phase 3 (6.7–6.5 Ma) this gateway shallowing reduced the oceanic input into the Mediterranean Basin causing significant hydrological changes, reflected in a wide range of temperature and salinity fluctuations accompanied by enhanced water-column stratification. The data presented here counterbalance the general lack of quantitative temperature and especially salinity estimates available for the Mediterranean Messinian, time interval where we still highly rely on modelling for such evaluations. This study highlights the severity of preconditioning stages leading to the Messinian Salinity Crisis in the Mediterranean and sets values for extreme salinity conditions that could still host marine life. • Three restriction phases affected the eastern Mediterranean between 7.2 and 6.5 Ma. • Colder water and normal salinity characterized the Mediterranean during 7.2–6.9 Ma. • Periods of strong evaporation observed for the 6.9–6.7 Ma interval. • Warm-hypersaline events (6.9–6.82 and 6.72 Ma) caused extreme environmental stress. • Restriction at 6.7–6.5 Ma was induced by tectonics and modulated by climate.

Investigation of subsidence phenomenon and impact of groundwater level drop on alluvial aquifer, case study: Damaneh-Daran plain in west of Isfahan province, Iran

The subsidence phenomenon is influenced by several factors, the most important of which is the overutilization of alluvial aquifers or reduced aquifer recharge as a result of climate change and drought. Alluvial Damaneh-Daran aquifer in central Iran is an unconfined aquifer. But due to its lithological diversity, behave similarly to a confined aquifer. Therefore, the aquifer is susceptible to subsidence. The subsidence in the study area was investigated using a hybrid numerical approach. Therefore, the rate of subsidence in the aquifer was first determined by the radar interferometer method. Then, to investigate the effecting parameters on subsidence, a groundwater model was constructed that showed a negative water budget. The fifty percent exploitation decrease scenario from the aquifer showed the significant and considerable effect of reducing pumping on the decrease in subsidence. Therefore, it was concluded that one of the primary and compelling reasons for the subsidence event in the Damaneh-Daran alluvial aquifer was the overexploitation of the aquifer. Given the lack of suitable and portable water resources in the catchment, changing the pattern of cultivation and optimal utilization of groundwater resources is a management strategy to control the current trend of overuse of aquifer and subsidence events.

Circulation and transport in short, low-inflow estuaries under anthropogenic stresses

Data from field experiments were used to assess, for the first time, hydrodynamic and transport processes in three shallow low-inflow estuaries of Northeast Brazil (Ceará State). The estuaries of the Cocó, Pacoti and Pirangi rivers exhibit anthropogenic stresses related to freshwater withdrawals and sewage discharges and morphological changes. Sampling was carried out during complete spring tidal cycles at each estuary during consecutive days on November 6, 7 and 8, 2010. November is the end of the dry period, and the results represented expected extreme low inflow conditions. All estuaries showed well mixed water columns because of frictional influences. Cocó and Pacoti displayed ebb dominance while the Pirangi exhibited flood dominance, a result from morphological changes in the intertidal storage areas. Each estuary showed distinct longitudinal salinity distributions related to different water budgets, which were actually altered by human activities. The Cocó estuary was hyposaline because of waste water discharge from the city of Fortaleza. This estuary represented a source of materials to the coastal sea as it showed net export of Suspended Particulate Matter (SPM) and chlorophyll. The Pacoti estuary was slightly hypersaline but nearly neutrally buoyant as temperature compensated for salinity variations. The Pacoti also exhibited net seaward transport of SPM and chlorophyll. The Pirangi estuary showed strong hypersalinity and inverse estuarine circulation as a result of multiple river dams. The negative water budget combined with mangrove depletion led to magnifying trapping processes, thus resulting in net import of salt, SPM and chlorophyll. The three cases exemplified how estuaries on the order 10 km long can be sensitive to human-induced changes. These human changes affect their circulation structure and their function as source or trap for materials at the land–ocean interface.

Variation of the mid-latitude westerlies and Indian summer monsoon since the last deglaciation: Evidence from grain-size data from Linggo Co, central Tibet

The mid-latitude westerlies and Indian summer monsoon (ISM) have a significant impact on the water budget and general environment of the Tibetan Plateau (TP), and much work has been conducted in order to characterize variations in these two atmospheric circulation systems on various time scales. However, the nature of the interplay between the Asian summer monsoon and the westerlies during the last deglaciation remains controversial. In this study, a sediment core from Linggo Co, a closed lake in the central TP, was analyzed for grain size and ostracod content. The results suggest that environmental change in the Linggo Co basin during the past 17 ka can be divided into four stages: during Stage 1 (17–11.7 ka) Linggo Co was a shallow lake characterized by high amplitude hydrological fluctuations. Strong winds prevailed in winter, correlative with cold events in the North Atlantic region, and indicating the dominance of the westerlies. During Stage 2 (~11.7 ka), in the early Holocene, Linggo Co expanded rapidly and there was the disappearance of the shallow-water ostracod species Limnocythere inopinata together with a decrease in mean grain-size. These changes indicate that ISM was enhanced and that it was the dominant atmospheric circulation affecting TP rather than the westerlies. During Stage 3 (10–8 ka) Linggo Co experienced several intervals with a negative water budget, which may be attributed to a weakening of the weaken ISM or to the reduced influx of glacial meltwater as a result of the influence of cold climatic events. During Stage 4, after 8 ka, Linggo Co shrank progressively in parallel with a gradual weakening of the ISM and finally a transition to the present environment occurred at ~2.5 ka. A comparison of the record from Linggo Co and other records from the TP reveals that prior to 16.5 ka the mid-latitude westerlies was the dominant atmospheric circulation system controlling the entire TP. From 16.5 to 11.5 ka, the mid-latitude westerlies still controlled the central-northern TP; however, the ISM controlled the southern TP, which had a similar climatic pattern to the present day. During the early-middle Holocene, the ISM was the dominant system affecting the entire TP; however, its influence was considerably reduced during the late Holocene, which was probably accompanied by a strengthening of the westerlies.

Impacts of rainfall changes on groundwater balance of coastal aquifers: a case study of the Thermaikos Gulf, North Greece

&lt;div&gt; &lt;p&gt;Groundwater is one of the major parameters in maintaining ecology in many regions. As climate is one of the main factors which affects groundwater resources, the main objective of the present study is to assess the impact of rainfall changes on the groundwater system by projecting the future changes in the 21&lt;sup&gt;st&lt;/sup&gt; century (2021-2050 and 2071-2100). For this reason, the RegCM3 climate model precipitation data, which showed a reduction in rainfall, was entered in the steady-state groundwater flow model MODFLOW for the case study of a coastal aquifer in the eastern part of the Thermaikos Gulf (North Greece). The uprising urbanization in combination with the intensive cultivation have led to the overexploitation of the coastal aquifer and seawater intrusion. The groundwater flow simulation by using the MODFLOW code indicates a negative water budget and estimates the quantities of the seawater intrusion. According to the RegCM3 climate model, the precipitation reduction is estimated to be 4% during the period of 2021-2050, while the precipitation decrease is expected to be 22% during the period of 2071-2100. Furthermore, the natural recharge of the coastal aquifer is expected to be influenced by the precipitation reduction. Finally, the seawater intrusion amounts are expected to increase during these future periods and more specifically during the second period of 2071-2100.&lt;/p&gt; &lt;/div&gt; &lt;p&gt;&amp;nbsp;&lt;/p&gt;

Recurrent phases of drought in the upper Miocene of the Black Sea region

Since the Miocene the Black Sea proved to be highly sensitive to fluctuations in the hydrological cycle because of its recurrent restricted connections with the open ocean and the location between the dry Mediterranean and more humid higher northern latitudes. Although the Black Sea formed one of the foci of the 1975 Deep Sea Drilling Project Leg 42B, robust tools to reconstruct past changes in its hydrological cycle were lacking at that time. Here we revisit the sedimentary succession (Hole 380A) and determined compound-specific carbon (δ13C) hydrogen isotope ratios (δD) of terrestrial and aquatic biomarkers to investigate changes in the hydrological budget of the Black Sea during the late Miocene. The δ13C and δD isotopic composition of n-alkanes as well as alkenones and palynology indicate large environmental changes in the Black Sea and/or in the sources of the water entering the Black Sea during the late Miocene. The δD of alkenones, showing an enrichment of more than 80‰ at the end of the Miocene, implies a major shift in basin hydrology, possibly resulting in severely increased salinity. These changes in δD composition of the alkenones coincide with both with sharp shifts in reconstructed sea surface temperature and palynological assemblages. Two intervals with negative water budget were identified, most likely caused by enhanced evaporation. The older and longer dry/evaporative phase predates the Maeotian/Pontian boundary (regional stages) at ~ 6.1 Ma. The younger drying phase corresponds to the Messinian Salinity Crisis. This shift to highly evaporative conditions is related to a similar shift previously observed in a Messinian (Pontian) sedimentary succession from the Taman Peninsula (Russia). These recurrent dry phases likely reflect regional climatic shifts over a significantly larger area around the Black Sea area during the upper Miocene.

A hydrological simulation of the water regime in two playa lakes located in southern Spain

The subject of this paper is the detailed hydrological simulation of two playa lakes located in southern Spain from January 2011 to March 2012 on a daily basis. These playas are placed over a 400-km 2 shallow aquifer, which is exposed to an increasing stress caused by agricultural activities, mainly olive grove plantations. The objective of the paper is to elaborate a detailed numeric model that simulates the water regime of each playa lake on a daily scale. The simulation is compared to measured water level (WL) data of the playas in order to characterize the groundwater–surface interactions. The ultimate objective of this paper is to assess the environmental impact of the increasing anthropogenic water consumption within the area of research. The results of the GW–surface interaction were very consistent with previous works. One of the playa lakes is groundwater-dependent and the other one is presumably a perched playa lake. The GW discharge of the former playa (214 mm) during the research period stands in sharp contrast to no regional GW discharge in the latter. Water level data prove that the hydrological year (2011–2012) had a very negative water budget. The evapotranspiration estimation was almost as high as double the sum of the precipitation, the run-off, and the groundwater discharge. The simulation of an anthropologically altered water regime proves that water retrieval has a harmful impact on the WL of the playa lakes as well as on the aquifer.

Climatic and environmental changes reflected by lake-level fluctuations at Gerzensee from 14,850 to 13,050 yr BP

High-resolution sediment analysis at Gerzensee, Swiss Plateau, focused on the reconstruction of lake-level changes during the last glaciation Greenland Stadial 2a (GS-2a) and a large portion of Lateglacial including the Gerzensee oscillation. The chronology is derived from a comparison of the oxygen-isotope stratigraphy established at Gerzensee with that of the NGRIP ice core. On a multi-millennial scale differences between the lake-level and oxygen-isotope records established from cores GEJ–GEK reflect a complex interplay among climate, vegetation and lake level during retreat of the ice sheet. But on a multi-centennial scale both the lake-level and oxygen-isotope records show a general agreement, i.e. cool periods such as Greenland Stadial GS-2a and Greenland Interstadials-1d and -1b, which coincided with more positive water budgets and, conversely, warm periods (Greenland Interstadials-1e and -1c) with more negative water budget. This is in agreement with the regional pattern of palaeohydrological changes reconstructed from previous studies. Despite possible reduced annual precipitation during GS-2a, the maintenance of relatively high lake-level conditions at Gerzensee at this time could have resulted from reduced evaporation and stronger runoff on bare slopes, whereas during the Lateglacial Interstadial the phases of lake-level lowering may have been associated with summer dryness linked to increasing summer insolation.

Organohalogen emissions from saline environments – spatial extrapolation using remote sensing as most promising tool

Abstract. Due to their negative water budget most recent semi-/arid regions are characterized by vast evaporates (salt lakes and salty soils). We recently identified those hyper-saline environments as additional sources for a multitude of volatile halogenated organohalogens (VOX). These compounds can affect the ozone layer of the stratosphere and play a key role in the production of aerosols. A remote sensing based analysis was performed in the Southern Aral Sea basin, providing information of major soil types as well as their extent and spatial and temporal evolution. VOX production has been determined in dry and moist soil samples after 24 h. Several C1- and C2 organohalogens have been found in hyper-saline topsoil profiles, including CH3Cl, CH3Br, CHBr3 and CHCl3. The range of organohalogens also includes trans-1,2-dichloroethene (DCE), which is reported here to be produced naturally for the first time. Using MODIS time series and supervised image classification a daily production rate for DCE has been calculated for the 15 000 km2 ranging research area in the southern Aralkum. The applied laboratory setup simulates a short-term change in climatic conditions, starting from dried-out saline soil that is instantly humidified during rain events or flooding. It describes the general VOX production potential, but allows only for a rough estimation of resulting emission loads. VOX emissions are expected to increase in the future since the area of salt affected soils is expanding due to the regressing Aral Sea. Opportunities, limits and requirements of satellite based rapid change detection and salt classification are discussed.

Early Holocene drought in the Laurentian Great Lakes basin caused hydrologic closure of Georgian Bay

Multiple proxies record aridity in the northern Great Lakes basin ~8,800–8,000 cal (8,000–7,200) BP when water levels fell below outlets in the Michigan, Huron and Georgian Bay basins. Pollen-climate transfer function calculations on radiocarbon-dated pollen profiles from small lakes from Minnesota to eastern Ontario show that a drier climate was sufficient to lower the Great Lakes, in particular Georgian Bay, to closed basins. The best modern climate analog for the early Holocene late Lake Hough stage in the Georgian Bay basin is Black Bass Lake near Brainerd MN. Modern annual precipitation at Brainerd is ~35% lower than at Huntsville ON, in the Georgian Bay catchment; warmer summers and colder, less snowy winters make Brainerd drier than the Georgian Bay snow belt. These values parallel transfer function reconstructions for the early Holocene from pollen records at five small lakes in the Georgian Bay drainage basin. Higher evaporation and evapotranspiration due to greater seasonality during the early Holocene produced a deficit in effective moisture in Georgian Bay that is recorded by the jack/red pine pollen zone that spanned ~8,800–8,200 cal (8,000–7,500) BP. This deficit drove late Lake Hough ~5 m below Lake Stanley in the Huron basin, following diversion of Laurentide Ice sheet meltwater from the Great Lakes basin. The level of Georgian Bay largely depends not on fluvial input from its own drainage basin, but rather from Lake Superior, where the early Holocene moisture deficit was greater. Reconstruction of paleoclimates in Minnesota, northwestern Ontario and Wisconsin produced a closed lake in the Superior basin, which removed the main water input to Georgian Bay. Once the inflow through the St. Marys River was reduced and inflow from other tributary streams was adjusted for isostatic and climatic differences, input was <5% of modern values. Consequent high evaporation rates produced a significant fall in lake level in the Georgian Bay basin and a negative water budget. This reduction in basin supply, together with the high conductivity of stagnant water in late Lake Hough inferred from microfossils in lowstand sediments, peaked at the end of the jack/red pine zone, ~8,300–8,200 (7,450 ± 90) BP. These major hydrologic changes resulting from climate change in the recent geologic past draw attention to possible declines of the Great Lakes under future climates.

Playas of the Thar Desert: Mineralogical and Geochemical Archives of Late Holocene Climate*

This study presents a synthesis of the published data and summarizes new data on mineralogy and geochemistry of the late Holocene playa sediments from the Thar Desert, located in the down stream of southwest monsoon. The eastern margin of the Desert is semi-arid (>400 mm a -1 ), whereas the western region is arid (200-300 mm a -1 ). The negative water budget (higher evapo-transpiration/precipitation) has led to the presence of a number of playas in the region. The assemblage of clastic minerals is similar and constitutes quartz, plagioclase, K-feldspar, amphibole, mica and chlorite. The non-clastic mineral assemblage is variable and comprises of both evaporite minerals and carbonates. The geochemical proxies enabled differentiation of the shallow depth profiles at five different playas into horizons of varying chemical weathering, aeolian input and evaporation. Between ca.1.3-3.1 ka, the dominance of proto-dolomite, [Ca >1 (Fe,Mg) (CO 3 ) 2 ] in the eastern playas and presence of gypsum (CaSO 4 .2H 2 O) in the western playas suggest that the present day climatic gradient might be exiting during the late Holocene.

Hydrogeology of the Kabul Basin (Afghanistan), part II: groundwater geochemistry

Shallow groundwater is the main source for drinking water in Kabul, Afghanistan. It comes from a multitude of shallow hand-pumped wells spread over the whole city area. The groundwater is characterised by slightly oxic redox conditions. Interactions with aquifer carbonates lead to near-neutral pH and high degrees of hardness. The mostly negative water budget of the Kabul Basin is the result of strong evaporation which leads to an increase in salt and also of some undesirable constituents, e.g. borate. Several years of drought have aggravated this problem. The shallow groundwater in the city has received tremendous amounts of pollution due to a lack of proper waste disposal and sewage treatment. Common indicators are elevated concentrations of nutrients such as nitrate and faecal bacteria. The high infant mortality can at least partially be attributed to the insufficient water hygiene. Acid generated during the mineralisation of the wastewater is hidden due to the strong pH buffering capacity of the groundwater system. Redox and pH conditions preclude significant mobilisation of trace metals and metalloids.

Magnetostratigraphic, biostratigraphic, and stable isotope stratigraphy of an Upper Miocene drill core from the Salé Briqueterie (northwestern Morocco): A high‐resolution chronology for the Messinian stage

We report a high‐resolution stable isotope, carbonate, magnetostratigraphic, and biostratigraphic record from a 175‐m drill core from the Salé Briqueterie, which is part of the Bou Regreg section in northwestern Morocco. The Salé drill core spans the interval from paleomagnetic Chron C4n partim to C3r (earliest Gilbert), which represents the time leading up to and including the isolation and desiccation of the Mediterranean (i.e., the Messinian salinity crisis). During Chrons C3An and C3Ar (6.935 to 5.894 Ma) the isotope and carbonate signals display quasi‐periodic variations with estimated periods of 40 and 100 kyr, respectively. We interpret the 40‐kyr δ18O variations as reflecting changes in global ice volume caused by obliquity‐induced changes (41 kyr) in solar insolation in polar regions. The 100‐kyr carbonate variations probably represent long‐term modulation of the amplitude of the precessional cycle (∼21 kyr), which is not resolved by our sampling frequency. The cyclic nature of the oxygen isotope signal permits us to extend the isotope nomenclature of Shackleton et al. (1994a) from stage TG24 in Chron C3r (earliest Gilbert) to stage C3Ar.δ18O.18 at the base of Chron C3Ar (6.935 Ma). A major change in paleoceanographic conditions is recorded across the Tortonian/Messinian boundary, which we correlate to Chron C3Bn at 7.04 Ma. Benthic foraminiferal δ18O values increased by an average of 0.4‰ in two steps at 7.17 Ma and 6.8 Ma and δ13C values decreased by 0.7–0.8‰ between 7.1 and 6.8 Ma, representing the late Miocene carbon shift. The first step in δ18O values coincides with an inferred reversal in deep water circulation through the Rifian Corridor, and the second correlates with the base of the Tripoli Formation and onset of “crisis conditions” in the Mediterranean. We suggest that the increase in δ18O values represents, at least in part, an increase in global ice volume that lowered sea level and contributed to the establishment of a negative water budget in the Mediterranean. Average δ18O values remained high throughout most of Chrons C3Ar and C3An, reaching maximum δ18O values during isotope stages TG20 and 22 in Chron C3r (earliest Gilbert). The glacio‐eustatic falls associated with these events may have resulted in the complete isolation of the Mediterranean from the world ocean (Shackleton et al., 1994a). Following stage TG12 in the Salé record, there exists a trend toward progressively lower δ18O values that may represent a series of marine transgressions that eventually reflooded the Mediterranean and ended the Salinity Crisis.

Radionuclide Transport above a Near‐Surface Water Table: II. Vertical Distribution of Gamma Activities within Soil Profiles in Relation to Wheat Rooting Density and Soil‐to‐Plant Transfers

AbstractRadiochemical results are described from a replicated field lysimeter experiment to investigate the upward migration of radionuclides in vegetated soils above near‐surface contaminated groundwater. Water tables were maintained at depths of 35 cm (shallow) and 65 cm (deep), below which a cocktail of radionuclides including 137Cs, 109Cd, 60Co, and 22Na was introduced. Results of the vertical migration and plant uptake are presented from the first 2 yr of an ongoing experiment with winter wheat (Triticum aestivum L.). As expected, 137Cs, 60Co, and 109Cd proved to be highly sorbed to the soil solid phase in the regions of the water table and exhibited a lesser degree of up‐profile transport than 22Na in shallow lysimeters. However, in deep lysimeters, 137Cs and 60Cs appeared to be preferentially accumulated at the soil surface; a biological translocation mechanism is postulated to account for this observation. It was clear from a statistical analysis of vertical radionuclide distributions that, in physicochemical terms, 22Na was the most mobile of the radionuclides studied. The distribution of this radionuclide within the soil profiles showed a significant shift between the two growth seasons examined, whereas the distributions of the other radionuclides remained static between 1990 and 1991. Each radionuclide showed a distinct and consistent distribution pattern throughout the crop tissues, although the differences in tissue specific activities for individual radionuclides were found to be nonsignificant with the exception of 22Na. Soil‐to‐plant transfer factors (TFw) were calculated using mean soil specific activities weighted according to the fractional abundance of roots and measured soil specific activities in discrete soil layers. The TFw values were significantly lower in 1991 than in 1990 in both shallow and deep lysimeters; it is postulated that this was due to a much greater fiux of water through the crop in 1990, which was characterized by high evapotranspiration rates and a substantial negative water budget. In both years, TFw values were, in general, significantly higher for deep lysimeters than for shallow lysimeters. This is thought to provide evidence of enhanced radionuclide absorption by the relatively small fraction of roots in the vicinity of the deeper water table.

Spatial patterns and frequency distribution of Late Quaternary water budget tendencies in Africa

Regional water budget tendencies reported by 14C-dated field data are correlated with Late Quaternary cooling and warming phases. Areal frequency distribution analysis of these correlations shows cooling phase negative and warming phase positive water budget tendency to be negatively correlated, with a significantly larger area reacting in cooling phases. Six main phases of climatic and environmental change are deduced from functional time and elevation plots. The only clear phase dislocations of positive and negative water budget tendency in all climatic zones are identified for the period before 23,000 yr B.P. and for the Full Glacial 20,000 to 17,000 yr B.P. There are clear differences in chronological sequences between the subtropics and tropics, but during Holocene very complex time and elevation patterns are apparent. This implies a strong cooling phase intensification of subtropical anticyclonic belts. The aridification gradient rises exponentially from subtropical arid to humid innertropical Africa wheras the humidification gradient shows a reverse structure. Humid tropical Africa is less susceptible to climatic change than semiarid regions and the arid zone is least affected. In those regions being susceptible to both cooling and warming trends non-climatic factors may play an important role in ecosystem fluctuations. Water budget tendency analysis does not indicate large-scale glacial and interglacial dislocations of continental atmospheric circulation patterns.