Total Seasonal Rainfall Research Articles

On the withdrawal of the Indian summer monsoon

AbstractThe withdrawal of the Indian summer monsoon (ISM) is defined in terms of 850 mb daily winds. The withdrawal of the ISM is found to be more variable than the onset. Most of the interannual variability of the total seasonal rainfall is dominated by the variability of the retreat phase. The withdrawal of the ISM follows a period of enhanced convective activity over the Indian subcontinent and is associated with a dry phase of the intraseasonal oscillation. The intraseasonal break dynamics have relatively more hemispheric symmetry than during the main monsoon season. The monthly persistence of interannual anomalies, the correlations with El Niño and remote upper‐level temperature signals of a Rossby type are all stronger during the withdrawal than in the summer months. These observations are found to be consistent with a mobile latent heat source migrating towards the west Pacific during the monsoon withdrawal. Copyright © 2004 Royal Meteorological Society.

Validation of TRMM and Other Rainfall Estimates with a High-Density Gauge Dataset for West Africa. Part I: Validation of GPCC Rainfall Product and Pre-TRMM Satellite and Blended Products

Gauge data over North Africa are used to provide an assessment of satellite and blended rainfall products for 1988‐94 and for 1998. A comparison is also made with the Global Precipitation Climatology Center (GPCC) gauge dataset. For the 1988‐94 period, mean fields and latitudinal transects for the June‐July‐August season are presented, based on a 515-station gauge dataset, the GPCC gauge data, the Global Precipitation Climatology Project (GPCP) blended data, the infrared-based Geostationary Operational Environmental Satellite precipitation index (GPI), and the Special Sensor Microwave Imager (SSM/I) microwave estimates. Error calculations are also presented. The mean fields derived from the dense gauge network, the GPCC gauge-only analysis, and the GPCP are remarkably similar. The bias, with reference to the seasonal rainfall field based on the denser network, is about 3%‐4% for either GPCC or GPCP. Agreement is relatively good, even in individual years. The rms error associated with these datasets is 12% for seasonal rainfall totals; thus, the error is largely random. In contrast, there are large systematic errors in the satellite-only analyses of GPI and SSM/I, with biases of 20% and 40% for the mean rain field as a whole and much larger biases in individual years. The rms errors are nearly 2 times as great. For 1998, a 920-station gauge dataset was available for a smaller section of West Africa. The comparison confirmed the superior performance of GPCP and demonstrated the lower level of performance of both GPCP and GPCC at the monthly scale as compared with the seasonal scale. Overall, the results of this study underscore the continued need for extensive gauge networks to describe adequately the large-scale precipitation field over Africa.

Prediction of East African Seasonal Rainfall Using Simplex Canonical Correlation Analysis

Abstract A linear statistical model, canonical correlation analysis (CCA), was driven by the Nelder–Mead simplex optimization algorithm (called CCA-NMS) to predict the standardized seasonal rainfall totals of East Africa at 3-month lead time using SLP and SST anomaly fields of the Indian and Atlantic Oceans combined together by 24 simplex optimized weights, and then “reduced” by the principal component analysis. Applying the optimized weights to the predictor fields produced better March–April–May (MAM) and September–October–November (SON) seasonal rain forecasts than a direct application of the same, unweighted predictor fields to CCA at both calibration and validation stages. Northeastern Tanzania and south-central Kenya had the best SON prediction results with both validation correlation and Hanssen–Kuipers skill scores exceeding +0.3. The MAM season was better predicted in the western parts of East Africa. The CCA correlation maps showed that low SON rainfall in East Africa is associated with cold SST...

TECHNICAL NOTE: First evidence of a structured and dynamic spatial pattern of rainfall within a small humid tropical catchment

Abstract. In a study of the spatial variability of rainfall across a network of 46 raingauges in a 4 km2 rainforest catchment in the interior of northeastern Borneo, seasonal rainfall totals were correlated with raingauge separation distance, aspect and relief. A very high degree of spatial variability in seasonal totals across a very small area was found, even in comparison with other regions experiencing convective rainfall. Moreover, it shows systematic, stochastic structure in rainfall is present over scales of 10s to 100s metres; these patterns change from the southwest monsoon (May-October) to the northeast monsoon (November-April). Local associations with aspect and relief are present but the seasonal changes in rainfall pattern over the whole 4 km2 catchment must relate to more complex local topographic effects on the regional windfield. Keywords: catchment, Malaysia,monsoon, rainfall, spatial variability

Influence of sea surface temperature anomalies in the Gulf of California on North American monsoon rainfall

Linkages between sea surface temperatures (SSTs) in the Gulf of California and the North American monsoon rainfall were examined using observational data and regional model experiments. SSTs in the Gulf start to increase in late June and reach a maximum in August. Monsoon onset dates in the Southwest were determined from the gridded rainfall analysis. Monsoon onsets can occur before SSTs in the Gulf reach a maximum. There is no relationship between monsoon rainfall onset dates or seasonal total rainfall over the Southwest and SSTs in the Gulf. Regional model experiments were then performed to study the impact of local SSTs on monsoon rainfall using the NCEP regional spectral model (RSM). RSM experiments were performed with the observed SSTs for four summers (July–September 1997–2000). The experiments were repeated with the same initial and boundary conditions but with the climatological SSTs in the Gulf of California and its vicinity. The model is able to capture the general features of monsoon rainfall and the diurnal cycle. Warm (cold) SSTs in the Gulf are responsible for more (less) rainfall along the western slopes of the Sierra Madre Occidental. Over the Southwest, the impact of local SSTs is small. Large‐scale flow has more influence on monsoon rainfall than SSTs in the Gulf of California because the SST forcing does not produce significant changes in the low level flow needed to influence rainfall over the Southwest.

Regression models for lentil seed and straw yields in Near East

Lentil ( Lens culinaris Medikus subsp. culinaris) is traditionally grown as a rainfed crop globally, except in Sudan and Egypt, where it is grown under supplemental irrigation. Its seed is a rich source of protein for human consumption, and the straw is a valued animal feed in Near East. In the Mediterranean environment, lentil productivity is limited primarily by the amount and distribution of rainfall and temperature extremes. This study aimed to develop simple models to predict lentil seed and straw yields as a function of seasonal weather data (rainfall and temperature). Historical weather and yield trial data over 39 environments from three contrasting locations in Syria and Lebanon were used. Best-fit models for seed and straw yields in terms of climatic variables based on stepwise regression were developed. These accounted for over 77% variance in yield for both models. The overall responses to total seasonal rainfall were 5.16 kg ha −1 mm −1 seed yield and 10.7 kg ha −1 mm −1 for straw yield. Although May (period of pod-filling) rain contributed substantially to seed yield, high May temperatures reduced yield drastically. For straw yield, absolute minimum temperature and February temperature are important factors, in addition to total seasonal rainfall. The simple regression models developed from historical data not only describe the variation in lentil yields but can also be used for yield prediction purposes in the Near East.

Intra‐seasonal rainfall characteristics and their importance to the seasonal prediction problem

AbstractDaily station rainfall data in South Africa from 1936 to 1999 are combined into homogeneous rainfall regions using Ward's clustering method. Various rainfall characteristics are calculated for the summer season, defined as December to February. These include seasonal rainfall total, region‐average number of station rain days exceeding 1 and 20 mm, region‐average of periods between rain days at stations >1 and >20 mm, region‐average of wet spell length (sequential days of station rainfall >1 and >20 mm), correlation of daily station rainfall within a region and correlation of seasonal station rainfall anomalies within a region.Rank‐ordered rainfall characteristic data generally form an s‐shaped curve, and significance testing of discontinuities in these curves suggests that normal rainfall conditions in South Africa consist of a combined middle three quintiles separated from the outer quintiles, rather than the traditional middle tercile.The relationships between the various rainfall characteristics show that seasons with a high total rainfall generally have a higher number of heavy rain days (>20 mm) and not necessarily an increase in light rain days. The length of the period between rain days has a low correlation to season totals, demonstrating that seasons with a high total rainfall may still contain prolonged dry periods. These additional rainfall characteristics are important to end‐users, and the analysis undertaken here offers a valuable starting point for seeking physical relationships between rainfall characteristics and the general circulation. Preliminary studies show that the vertical mean wind is related to rainfall characteristics in South Africa. Given that general circulation models capture this part of the circulation adequately, seasonal forecasts of rainfall characteristics become plausible. Copyright © 2002 Royal Meteorological Society.

Selection for Reduced Carbon Isotope Discrimination Increases Aerial Biomass and Grain Yield of Rainfed Bread Wheat

Genetic gain is characteristically slow when selecting directly for increased grain yield under water‐limited conditions. Genetic increases in grain yield may be achieved through increases in aerial biomass following selection for greater transpiration efficiency (TE as aerial biomass/water transpired). Strong negative correlations between TE and carbon isotope discrimination (Δ) in wheat (Triticum aestivum L.) suggest that selection of progeny with low Δ may increase TE and aerial biomass under water‐limited conditions. This study investigated how early generation, divergent selection for Δ affected aerial biomass and grain yield among 30 low‐ and 30 high‐Δ, ‘Hartog’‐like, BC2F4:6 progeny and the recurrent, high‐Δ parent Hartog. Lines were evaluated in nine environments varying for seasonal rainfall (235–437 mm) and hence grain yield (1.3–6.2 Mg/ha). Selection for low Δ in early generation progeny was associated with significantly (P < 0.01) smaller Δ, higher grain yield (+5.8%), aerial biomass (+2.7%), harvest index (+3.3%), and kernel size (+4.8%) in tested lines. Kernel number was the same for low‐ and high‐Δ selected groups. Grain yield advantage of the low Δ group increased with reductions in environment mean yield (r = −0.89, P < 0.01) and total seasonal rainfall (r = −0.85, P < 0.01) indicating the benefit of low Δ, and therefore high TE for genetic improvement of grain yield in lower rainfall environments. Narrow‐sense heritability on a single‐plot basis was much greater for Δ (h2 = 0.63 ± 0.10) than for either aerial biomass (0.06 ± 0.05) or grain yield (0.14 ± 0.04). Strong genetic correlations between Δ and both aerial biomass (r g = −0.61 ± 0.14) and grain yield (−0.58 ± 0.12) suggest Δ could be used for indirect selection of these traits in early generations. Selection of low Δ (high TE) families for the advanced stages of multiple‐environment testing should increase the probability of recovering higher‐yielding wheat families for water‐limited environments.

Selection for Reduced Carbon Isotope Discrimination Increases Aerial Biomass and Grain Yield of Rainfed Bread Wheat

Genetic gain is characteristically slow when selecting directly for increased grain yield under water-limited conditions. Genetic increases in grain yield may be achieved through increases in aerial biomass following selection for greater transpiration efficiency (TE as aerial biomass/water transpired). Strong negative correlations between TE and carbon isotope discrimination (Δ) in wheat (Triticum aestivum L.) suggest that selection of progeny with low Δ may increase TE and aerial biomass under water-limited conditions. This study investigated how early generation, divergent selection for Δ affected aerial biomass and grain yield among 30 low- and 30 high-Δ, ‘Hartog’-like, BC2F4:6 progeny and the recurrent, high-Δ parent Hartog. Lines were evaluated in nine environments varying for seasonal rainfall (235–437 mm) and hence grain yield (1.3–6.2 Mg/ha). Selection for low Δ in early generation progeny was associated with significantly (P < 0.01) smaller Δ, higher grain yield (+5.8%), aerial biomass (+2.7%), harvest index (+3.3%), and kernel size (+4.8%) in tested lines. Kernel number was the same for low- and high-Δ selected groups. Grain yield advantage of the low Δ group increased with reductions in environment mean yield (r = −0.89, P < 0.01) and total seasonal rainfall (r = −0.85, P < 0.01) indicating the benefit of low Δ, and therefore high TE for genetic improvement of grain yield in lower rainfall environments. Narrow-sense heritability on a single-plot basis was much greater for Δ (h2 = 0.63 ± 0.10) than for either aerial biomass (0.06 ± 0.05) or grain yield (0.14 ± 0.04). Strong genetic correlations between Δ and both aerial biomass (r g = −0.61 ± 0.14) and grain yield (−0.58 ± 0.12) suggest Δ could be used for indirect selection of these traits in early generations. Selection of low Δ (high TE) families for the advanced stages of multiple-environment testing should increase the probability of recovering higher-yielding wheat families for water-limited environments.

Teleconnections between the tropical Pacific and the Sahel

AbstractA greater understanding of equatorial teleconnections is a key aspect of research into seasonal prediction and future climate change for tropical regions. Here the impact of Pacific sea surface temperature (SST) anomalies on rainfall fluctuations over the semi‐arid Sahel is explored, using a combination of observed and model data.The first key topic of this study is the identification of those aspects of anomalous Pacific SST variability that are most strongly linked to the Sahel. One of these, also illuminated by earlier studies, is similar to the classic El Niño Southern Oscillation pattern in the central and east Pacific, which in its El Niño phase increases the likelihood of Sahel drought. It is shown here that, although a part of this link is indirect (operating via Atlantic SSTs), its main effect appears to be through a direct atmospheric teleconnection. The other critical pattern, of equal importance and revealed here by a novel analysis technique, is the large‐scale zonal gradient of SSTs from the west Pacific to the east Indian Ocean. If weakened, this too enhances the likelihood of Sahel drought. Atmospheric general circulation model experiments, forced either by observed or idealized SSTs, are used to confirm these two influences on the Sahel. Crucially, their Sahelian impact is substantially reinforced when both are present and, additionally, further empirical analysis shows them to be largely independent.The second key topic is an investigation of the mechanisms for this Pacific‐Sahel teleconnection. These appear to involve anomalous stationary equatorial waves, with communication occurring in both the eastward and westward directions. In El Niño years (for example), a Kelvin wave emanates across the Atlantic from east Pacific convective heating anomalies, and an equatorial Rossby wave appears over the Indian Ocean in response to the anomalous west Pacific‐Indian Ocean SST gradients via convective heating anomalies over the Indian Ocean. These interact over Africa to enhance large‐scale subsidence over the Sahel, thus reducing seasonal rainfall totals. Interannual changes in propagating equatorial waves or in the residence of subseasonal regimes appear not to play a substantial role.

Global warming and monsoon climate

The response of the Asian summer monsoon to transient increases of greenhouse gases (GHGs) and sulfate aerosols in the Earth's atmosphere is examined using the data generated in numerical experiments with available coupled atmosphere-ocean global climate models (A-O GCMs). A comparison of observed and model-simulated trends in monthly mean near-surface temperature and rainfall over the region provides evidence of skill of the A-O GCMs in simulating the regional climatology. The potential role of the sulfate aerosols in obscuring the GHG- induced warming over the Indian subcontinent is discussed. Even though the simulated total seasonal rainfall over the Indian subcontinent during summer monsoon season is underestimated in most of the A-O GCMs, the year to year variability in simulated monsoon rainfall over the study region is found to be in fair agreement with the observed climatology.

Precipitation over Sardinia (Italy) during the 1946-1993 rainy seasons and associated large-scale climate variations

The interannual and decadal variability of seasonal precipitation (October through April) over Sardinia (Italy) is analysed for the years 1940–1995. It is shown that the area-average seasonal rainfall total can vary greatly from one year to the next. The spatial correlation of Sardinia seasonal rainfall with that of neighbouring Mediterranean regions is generally high, though the correlation is less to the south of Sardinia, suggesting other teleconnection structures may operate there. Over a slightly shorter period (1946–1993), correlations between precipitation and large-scale variables (sea level pressures (SLP), 500 hPa geopotential heights (Z500) and sea surface temperatures (SST)) are calculated and analysed. Clear connections between the North Atlantic Oscillation (NAO) and Sardinian precipitation are found and studied. Contrasting Atlantic storm tracks were found for the wet and dry seasons. In the wet years, the track is weaker over the North Atlantic (consistent with the NAO association), while over Europe there emerges a second minor storm track that enters the Mediterranean basin. In dry years this latter feature is less evident and is diverted to the lee of Sardinia, probably because of stronger intensity of the Azores High. Some apparent associations with other Northern Hemisphere remote teleconnections and large-scale structures also emerge, including the North Pacific and across North Africa. Copyright © 2000 Royal Meteorological Society

The influence of tillage on semi-arid soil–water regimes in Zimbabwe

Little is known about the in situ hydrological properties of soils in the semi-arid regions of the world where subsistence agriculture is the common land use. The work reported here concentrates on three major soil types, considered representative of the agriculturally important Zimbabwean fersiallitic group of soils, and describes the water regimes that develop under conventional (flat) and improved (tied ridge) practices in response to natural and simulated rainfall. The first soil, typical of those used by small scale farmers, is a deep coarse-grained granitic sand with a low available water capacity. The second is a silty clay loam derived from mafic rocks and typical of the red clays associated with Zimbabwe's commercial farming areas. The third is a sandy loam that readily compacts and crusts under natural rainfall, prone to runoff and thus drought sensitive. The climate at each site has a five month rainy season, when most of the rain falls as heavy convectional storms and a dry spell from April to October. However, total seasonal rainfall at each site is unreliable (200 to 1100 mm) with unpredictable dry periods of 1–3 weeks, especially at the beginning and middle of the wet season when rain occurs as intense storms of short duration. Long term monitoring of the three soil types has shown that at the end of the dry season, the quantity of water stored in the top 0.3 m of the profile is negligible, typically below permanent wilting point. Water stored in the 0.9 m profile at the end of the dry season is more variable and depends on total seasonal rainfall, its distribution, the type of crop grown and the tillage management. At the beginning of the wet season, the recharge of the soil profile is of fundamental importance for subsequent crop establishment and can benefit from improved soil water storage in the previous season and tillage practices that help to catch the first rains of the wet season. In general terms the tillage treatment that impounds the most water (tied ridges) was the wettest, whatever the soil type. However, the incomplete wetting up of the tied ridges early in the wet season, frequently resulted in poor crop establishment or a delay in planting until the ridge structure was fully wetted. This was in contrast to the conventionally tilled soils, which were more uniformly wet in the top 200 mm of the soil profile and significantly wetter than the ridged soils early in the wet season. Irrespective of tillage technique, good weed management was critical to maximize the availability of water for crop growth.

An evaluation of three stochastic rainfall models

Three stochastic rainfall models are evaluated in terms of their ability to reproduce the standard and extreme statistics derived from observed hourly point rainfall data obtained from three different sites in the UK. The models are: a modified version of the Eagleson [Eagleson, P.S., 1972. Dynamics of flood frequency. Water Resources Research 8, 878–898] exponential model (or MEEM), a version of the data-based model of Cameron et al. [Cameron, D.S., Beven, K.J., Tawn, J., Blazkova, S., Naden, P., 1999. Flood frequency estimation for a gauged upland catchment (with uncertainty). Journal of Hydrology 219, 169–187] (or CDFGPDM) and the random parameter Bartlett–Lewis gamma model (or RPBLGM) of Onof and Wheater [Onof, C., Wheater H.S., 1994. Improvements to the modelling of British rainfall using a modified random parameter Bartlett–Lewis rectangular pulse model. Journal of Hydrology 157, 177–195]. The sites are: Elmdon (Birmingham, England), Eskdalemuir (White Esk valley, Scotland), and the Wye (Plynlimon, Wales). For each site, the simulations are conducted on a seasonal (winter and summer) basis, with hourly timestep. For each season, a comparison of the spread of the 1 and 24 h simulated seasonal maximum rainfall totals with those obtained through a statistical analysis of the observed extreme rainfalls is also made. It is shown that the MEEM can effectively reproduce certain observed series standard statistics at each site. It is much poorer in its representation of extreme events, and, for two of the sites, dry periods. The CDFGPDM generally performs well under all the criteria, although it has a tendency to underestimate the observed 1 h seasonal maximum rainfalls for both seasons at Elmdon. The RPBLGM is shown to be reasonable at simulating the standard statistics but is often poorer with respect to the extremes. The implications for flood frequency estimation are highlighted. Possible improvements to each model are considered.

Monsoon rainfall and its variability in Godavari river basin

Rainfall variability over a river basin has greater impact on the water resource in that basin. With this in view, the variability of the monsoon rainfall over the Godavari river basin has been studied on different time scales. As expected, the monsoon rainfall in Godavari basin is more variable (17%) than the all-India monsoon rainfall (11%) during the period of study (1951–90). Similarly, inter-annual variability of the monsoon rainfall on smaller time scales is found to be still higher and increases while going on from seasonal to daily scales. An interesting observation is that the intra-seasonal variability of the monsoon rainfall has a significant negative relationship (CC= −0.53) with the total seasonal rainfall in the basin.

Spatio-temporal variations in thunderstorm rainfall over Nigeria

Thunderstorm rainfall is a major form of convective rainfall over Nigeria. Its contribution to total wet season rainfall increases from about 18% in the south to 36% in the north. The importance of thunderstorms lies in the fact that they contribute significantly to flooding episodes and soil erosion processes due to their high intensity and torrential characteristics. This paper examines the spatial and temporal variations in thunderstorm rainfall over Nigeria using daily rainfall data and associated weather information over a 30 year period (1960–1989) for 19 synoptic stations. Results show that the dry season distribution pattern of rainfall due to thunderstorms shows a general decrease from south to north similar to the total rainfall distribution pattern in the country. At the peak of the wet season (July–September) thunderstorm rainfall increases northwards up to around latitude 11°N when it begins to decrease. The effect of orography in enhancing thunderstorm rainfall across the country is also shown. Elementary linkage analysis was used to group the 19 synoptic stations into thunderstorm rainfall regions. Six regions were identified which showed internal coherence in terms of temporal fluctuations of thunderstorm rainfall. © 1998 Royal Meteorological Society

On the characteristics of the rainfall events in the Sahel with a view to the analysis of climatic variability

The lack of appropriate data has long been a major obstacle to the study of the Sahelian rainfall at the event scale. In this paper, use is made of the EPSAT-Niger recording rain-gauge data to characterize the convective rain events of the central Sahel. Although some considerations lead to the identification of two main types of mesoscale convective systems, it is shown here that the most relevant stratification in terms of statistical analysis of the event rainfall distribution is between the events of the margins and those of the core of the rainy season. In fact, the average storm rain-depth appears to be non-stationary in time, with storm rain-depths slightly higher in the core of the rainy season than on the margins. Separation between the core and the margins thus allows the fitting of an exponential model to the observed storm rain-depth distributions of each period (core and margins), although a better fit would certainly be obtained if a proper modelling of the time non-stationarity was carried out. It is then shown that there is little, if any, correlation between the mean storm rain-depth of a given year and the overall abundance of the corresponding rainy season. This is a validation of previous works, which reached the same conclusion using daily rainfall data only. One major result of this work is that the statistics characterizing the rain events in the Sahel display little fluctuations, either in space or from year-to-year, as compared with those observed for the total seasonal rainfall. Each year and at each station the average storm rainfall remains close to 12 mm during the margins of the rainy season and close to 15 mm in the core. During the same period, the average seasonal rainfall over the study area ranged from 400 to 660 mm and for any given year the ratio between the maximum and the minimum point seasonal rainfall was of the order of 2. It is therefore concluded that the main source of rainfall variability in the Sahel is linked to the variability in the number of events rather than in the magnitude of these events. © 1998 Royal Meteorological Society

Nitrogen and water effects on wheat yield in a Mediterranean-type climate: I. Growth, water-use and nitrogen accumulation

Available soil water is the principal factor that limits the yield potential of wheat ( Triticum aestivum L.) and its response to nitrogen (N) under Mediterranean climatic conditions. We examined wheat yield, N fertilizer uptake, and water use for two seasons (1991/1992 and 1992/1993) with total seasonal rainfall of 323 and 275 mm, respectively. In addition to natural rainfall, supplemental irrigation at various rates (1/3, 2/3 and full irrigation) was also considered in assessing these parameters and their interactive effects. Grain yield and dry matter accumulation and N uptake varied between years, mainly due to rainfall and sowing date. Yields were increased by irrigation and N fertilization, with a greater response to N in 1991/1992 and to irrigation in the relatively drier 1992/1993 season. Most fertilizer N was taken up from tillering to anthesis; then it either stabilized or slightly declined, while soil N contributed further to plant N uptake. Compared to rain-fed conditions, more soil and fertilizer N was utilized by the irrigated crop, particularly in 1992/1993. In both years, grain N represented 60% of N accumulated at anthesis under rain-fed conditions, compared to 80% under irrigation. Fertilization and irrigation increased water-use efficiency, particularly under drier conditions. Thus, supplemental irrigation, applied at a sensitive growth stage, would be a valuable management practice for improving yield, water-use efficiency and crop N uptake under the dry conditions of a Mediterranean climate. Given the potential of both irrigation and N to increase output in dry areas, the amounts of both inputs are dictated by rainfall in any one year.

Simulation of the Indian monsoon climatology in ECHAM3 climate model: sensitivity to horizontal resolution

The ability of the current generation of climate models, in their long-term simulations, to replicate the observed atmospheric behaviour on a wide range of spatial and time scales provides support in applying these models to the greenhouse gas-induced climate change projections on regional scales. A series of long time-slice control experiments have been performed recently with the ECHAM3 atmospheric general circulation model at T21, T42 and T106 horizontal resolutions (30 years each with T21 and T42 and 5 years with T106). All these model experiments use a common sea-surface temperature climatology in their control experiment inferred from a coupled ocean–atmosphere climate model (ECHAM1+LSG) experiment. In this paper, we examine the ability of the ECHAM3 model to simulate the Indian monsoon climatology at these three different horizontal resolutions. Because the Indian summer monsoon circulation evolves through a characteristic sequence of events, it is important that the climate models should be able to realistically portray these important features of the circulation over adequate spatial and time scales. We focus on the model's simulation of selected variables representative of the thermal, dynamic and hydrological components in zonal mean cross- sections and area-averaged monthly as well as seasonal regional distributions. Generally, with respect to large-scale features of the circulation, the largest differences among the simulations occur at T42 relative to T21. At both T21 and T42 horizontal resolution, however, the model does not have a high degree of correspondence with observations as regards the spatial distribution of mean sea-level pressure, surface air temperature, 850 hPa winds and precipitation. On regional scales, T106 resolution best captures both the spatial and temporal characteristics of the Indian climatology. Both the diurnal and seasonal cycles of area-averaged surface air temperature over the region simulated by the model at T106 resolution are within 1 to 2°C as compared with observed climatology. The development and migration of the monsoon trough over Central India and the adjoining Bay of Bengal during the monsoon season is best simulated at T106 resolution. There is a distinct improvement in the spatial distribution as well as the total area-averaged summer monsoon rainfall in the model simulations with finer resolution. Although the modelled and observed mean summer precipitation is similar in overall structure at T106 resolution, underestimation of the total seasonal rainfall in the model even at high resolution is a reflection of the sensitivity of simulated precipitation to local climate forcings, e.g. tropical convergence zone, and deficiencies of parameterization schemes for convection and land surface processes. © 1997 by the Royal Meteorological Society. Int. J. Climato., 17: 847–858 (1977) (No. of Figures: 8. No. of Tables: 1. No. of

Underground water use by Eucalyptus trees in an arid climate

Expanding the area of tree plantations in the Negev desert of Israel requires prior quantification of the water resources in small watersheds. Combined hydrological and physiological measurements were used to study a “liman” (floodwater irrigated plot) in the Negev highlands, where Eucalyptus and other tree species are grown successfully. The amount of water flowing into the liman, surplus water flowing out of the liman, temporal soil moisture distribution, and water uptake by the trees were determined. Depending on rainfall intensity and distribution during the season, the liman received 2 – 3 times the total seasonal rainfall. Although the study was conducted during a year with a negligible amount of rainfall, the transpiration rate was closely correlated with potential transpiration throughout the year. The amount of water extracted from the soil was less than the time-integrated transpiration rate from the trees, suggesting that a water source other than soil water storage was available to the trees. We suggest that the trees extracted water from the rock fractures and/or utilized the lateral flows over the rock/soil interface.