Soil Moisture Potential Research Articles

Assessing Landscape Change and Population‐Environment Interactions via Panel Analysis

Landscape change involving landuse/landcover (LULC) traditionally was considered the territory of remote sensing, a result of clustering of spectral responses and subsequent interpretation. However, a more integrative approach to landscape change has evolved into a spatially explicit and scale-dependent practice more appropriately couched in the context of Geographic Information Science (GISc). GISc promotes the importance of combining multiple data sources and techniques into a scientifically grounded information system with an integrative and derivative capacity for characterizing scale, pattern, and process variables across thematic and spatial/temporal domains. Combining GISc and landscape ecology provides a meaningful way to interrelate theory and practice in the analysis of scale, one of the most fundamental and inherently geographic factors in assessing landscape form, function, and change. This research models the relationship between 25-years of landscape change and variables derived in a Geographic Information System (GIS) representing population, environmental gradients, and accessibility. These relationships are statistically tested across spatial scales to assess human-environment interactions at work on the landscape. In addition, particular attention is given to the characterization of multitemporal LULCC (LULC change) over the landscape at the pixel level (900 m landscape cells) as well as the characterization of forces at work on the landscape via correlation analysis and tests of significance across spatial scales to discern population-environment interactions. Pattern metrics are used to examine the composition and spatial organization of the landscape across spatial scales by computing selected indicators of LULC structure from the panel change histories derived through an expert system approach, defining the emergent properties of LULC dynamics across space and time. Lastly, three measures of spatial autocorrelation are assessed relative to landscape trajectories. The basic intent of the research is to study humanenvironment relationships in Nang Rong district in northeast Thailand by examining the propensity of LULCC in support of agricultural extensification through deforestation as a consequence of 1) total population at the village level, 2) resource endowment reflected in soil moisture potential, and 3) access to critical infrastructure for agriculture. To this end, landscape classification trajectories are created to measure agricultural extensification, deforestation, and LULC stability.

Spatial and temporal variation of fire regimes in a mixed conifer forest landscape, Southern Cascades, California, USA

AimIn this study, we evaluated the fire‐forest mosaic of a mixed conifer forest landscape by testing the hypothesis that pre‐fire suppression fire regime parameters vary with species composition (tree species), and environment (i.e. slope aspect, slope position, elevation).LocationOur study was conducted in the 1587 ha Cub Creek Research Natural Area (CCRNA), Lassen National Forest, CA, USA.MethodsWe quantified the return interval, seasonal occurrence, size, rotation period, and severity of fires using dendroecology.ResultsSlope aspect, potential soil moisture, forest composition, and fire regime parameters in our study area co‐vary. Median composite and point fire return intervals (FRI) were longest on higher, cooler, more mesic, north‐facing (NF) slopes covered with white fir (Abies concolor), Douglas fir (Pseudotsuga menziesii)–white fir, and red fir (A. magnifica)–white fir forests, shortest on the dry, south‐facing (SF) slopes covered with ponderosa pine (Pinus ponderosa)–white fir forests and intermediate on west‐facing slopes dominated by white fir–sugar pine (P. lambertiana)–incense cedar (Libocedrus decurrens) forests. The spatial pattern for length of fire rotation (FR) was the same as that for FRI. Fires in CCRNA mixed conifer forests occurred mainly (90%) in the dormant season. Size of burns in CCRNA mixed conifer forests were generally small (mean=106 ha), however, during certain drought years widespread fires burned across fuel breaks and spread throughout the watershed. Fire severity was mainly high on upper slopes, low on lower slopes and moderate and low severity on middle slopes. Patterns of fire severity also varied with slope aspect. Fire frequency decreased dramatically in CCRNA after 1905.ConclusionsIn CCRNA, fire regime parameters [e.g. FRI, fire extent, FR, fire severity] varied widely with species composition, slope aspect and slope position. There was also temporal variation in fire extent with the most widespread fires occurring during drought years. The important contributions of topography and climate to variation in the fire regime indicates that exogenous factors play a key role in shaping the fire‐forest structure mosaic and that the fire‐forest structure mosaic is more variable, less predictable and less stable than previously thought. Finally, some characteristics of the fire regime (i.e. fire severity, season of burn) in CCRNA are different than those described for other mixed conifer forests and this suggests that there are geographical differences in mixed conifer fire regimes along the Pacific slope.

Investigating the potential for soil moisture and surface roughness monitoring in drylands using ERS SAR data

Multitemporal ERS-1 and ERS-2 SAR data were acquired for northern Jordan between 1995 and 1997 to investigate changes in the backscatter coefficients of a range of typical desert land surfaces. The changes in backscatter found were ascribed to variations in surface soil moisture, and changes in surface roughness caused by a range of natural and anthropogenic factors. Data collected from monitored sites were input into the Integral Equation Model (IEM). The model outputs were strongly correlated with observed backscatter coefficients (r 2=0.84). The results show that the successful monitoring of soil moisture in these environments is strongly dependent on the surface roughness. On surfaces with RMS height 0.5 cm, the sensitivity of the backscatter coefficient to changes in surface microtopography did not allow accurate soil moisture estimation. Microtopographic change on rougher surfaces has less influence on the backscatter coefficient, and the probability of soil moisture estimation from SAR imagery is g...

Gradient Analysis of Fire Regimes in Montane Forests of the Southern Cascade Range, Thousand Lakes Wilderness, California, USA

Species distribution and abundance patterns in the southern Cascades are influenced by both environmental gradients and fire regimes. Little is known about fire regimes and variation in fire regimes may not be independent of environmental gradients or vegetation patterns. In this study, we analyze variation in fire regime parameters (i.e., return interval, season, size, severity, and rotation period) with respect to forest composition, elevation, and potential soil moisture in a 2042 ha area of montane forest in the southern Cascades in the Thousand Lakes Wilderness (TLW). Fire regime parameters varied with forest composition, elevation, and potential soil moisture. Median composite and point fire return intervals were shorter (4-9 yr, 14-24 yr) in low elevation and more xeric white fir (Abies concolor)-sugar pine (Pinus lambertiana) and white fir-Jeffrey pine (P. jeffreyi) and longest (20-37 yr, 20-47 yr) in mesic high elevation lodgepole pine (Pinus contorta) and red fir (Abies magnifica)-mountain hemlock (Tsuga mertensiana) forests. Values for mid-elevation red fir-white fir forests were intermediate. The pattern for fire rotation lengths across gradients was the same as for fire return intervals. The percentage of fires that occurred during the growing season was inversely related to elevation and potential soil moisture. Mean fire sizes were larger in lodgepole pine forests (405 ha) than in other forest groups (103-151 ha). In contrast to other parameters, fire severity did not vary across environmental and compositional gradients and >50% of all forests burned at high severity with most of the remainder burning at moderate severity. Since 1905, fire regimes have become similar at all gradient positions because of a policy of suppressing fire and fire regime modification will lead to shifts in landscape scale vegetation patterns.

Variations in Soil Moisture Content in a Rangeland Catchment

Soil water studies for California rangelands have focused on near-surface hydrologic processes, limiting our understanding of spatial-temporal dynamics of the water regime below the root zone. Soil moisture content and potential were monitored for 16 months in 12 locations in an annual grass dominated 20 ha catchment. The data collected were analyzed by ANOVA to determine significant spatial and temporal differences in soil moisture. Further analysis identified variables that influenced the amount of moisture present at a particular subsurface location. It was determined that there were significant differences in the amount of soil moisture present along the vertical profile of each site and between sites. Soil texture, type of vegetation cover, and elevation were the significant variables that influenced the soil moisture status.

Variations in soil moisture content in a rangeland catchment

Soil water studies for California rangelands have focussed on near-surface hydrologic processes, limiting our understanding of spatial-temporal dynamics of the water regime below the root zone. Soil moisture content and potential were monitored for 16 months in 12 locations in an annual grass dominated 20 ha catchment. The data collected were analyzed by ANOVA to determine significant spatial and temporal differences in soil moisture. Further analysis identified variables that influenced the amount of moisture present at a particular subsurface location. It was determined that there were significant differences in the amount of soil moisture present along the vertical profile of each site and between sites. Soil texture, type of vegetation cover, and elevation were the significant variables that influenced the soil moisture status.

Impact of documented land use changes on the surface albedo and evapotranspiration in a plain watershed

Agricultural land use series are investigated in a plain catchment area of the Tisza River, almost identically represented by six administrative counties. Each county, commonly covering 34,000 km2, is characterised by high percentage (70–80 %) of managed vegetation. Effects of area coverage variations between the different plants are computed for the period 1951–1993 by applying results of a literature-based syntheses, specified for Hungary. The latter studies estimate surface-albedo values and proportion between real and potential evapotranspiration for the great majority of the plants grown in the region. Potential evapotranspiration and relative soil moisture content are estimated from the monthly meteorological series of temperature, precipitation and atmospheric water vapour pressure. Product of these plant-specific characteristics and the relative area coverage yield in monthly series of surface albedo and real evapotranspiration. Furthermore, these values are related to the energy balance of the surface-atmosphere system by using a radiative-convective model adjusted for the given location. Two questions are investigated:i) Are there monotonous trends in the given terms of the energy and water budget?ii) Are these changes comparable to the effects caused by other external forcings?Our computations give positive answer to both questions.

Investigating the potential for soil moisture and surface roughness monitoring in drylands using ERS SAR data

Multitemporal ERS-1 and ERS-2 SAR data were acquired for northern Jordan between 1995 and 1997 to investigate changes in the backscatter coefficients of a range of typical desert land surfaces. The changes in backscatter found were ascribed to variations in surface soil moisture, and changes in surface roughness caused by a range of natural and anthropogenic factors. Data collected from monitored sites were input into the Integral Equation Model (IEM). The model outputs were strongly correlated with observed backscatter coefficients (r 2=0.84). The results show that the successful monitoring of soil moisture in these environments is strongly dependent on the surface roughness. On surfaces with RMS height 0.5 cm, the sensitivity of the backscatter coefficient to changes in surface microtopography did not allow accurate soil moisture estimation. Microtopographic change on rougher surfaces has less influence on the backscatter coefficient, and the probability of soil moisture estimation from SAR imagery is greater. These results indicate that knowledge of the surface conditions (both in terms of surface roughness and geomorphology) is essential for accurate soil moisture monitoring, whether in a research or operational context. The potential benefits of these findings are discussed in the context of the Jordan Badia Research and Development Project.

Water deficit effects on sweet corn. II. Canopy development

The response of sweet corn canopy development to timing and severity of water deficit was determined in an experiment using a mobile rainshelter. Six irrigation treatments were applied such that plots experienced: (1) no water deficit; (2) full water deficit; (3) moderate pre-silking deficit; (4) severe pre-silking deficit; (5) moderate post-silking deficit; or (6) severe post-silking deficit. Soil moisture content profiles were measured using neutron moisture probes and water deficit was quantified using the concept of ‘potential soil moisture deficit’ (Dp), which was calculated from climatic data. Water deficit reduced leaf area, and the effect varied depending on its timing and severity. Early water deficit reduced the rate of leaf expansion (but not duration of expansion) and, consequently, the maximum area of individual leaves; it therefore reduced the maximum leaf area index (LAI). Moderate post-silking water deficit did not significantly reduce maximum LAI, but it hastened leaf senescence. Severe water deficit late in crop growth reduced canopy development by decreasing maximum LAI and advancing leaf senescence. A model was developed which accounted for these effects. Two measures of Dp were defined which related to both short-term transient responses and those integrated over longer times. The model made it possible to simply and mechanistically describe the effects of water deficit on canopy development.

Water deficit effects on sweet corn. I. Water use, radiation use efficiency, growth, and yield

The responses of sweet corn biomass and yield to timing and severity of water deficit were determined in an experiment using a mobile rainshelter. Six irrigation treatments were applied such that plots experienced: (1) no water deficit; (2) full water deficit; (3) moderate pre-silking deficit; (4) severe pre-silking deficit; (5) moderate post-silking deficit; or (6) severe post-silking deficit. Drought was quantified using the concept of potential soil moisture deficit, which was calculated from climatic data. Potential soil moisture deficit can be related simply to a wide range of plant performance variables, making it possible to compare the relative importance of variables in determining the overall response of the crop to drought. For all treatments, yield was related strongly to biomass, especially that accumulated after silking. Biomass, in turn, was reduced by water deficit, mainly because of reduced radiation use efficiency, but also because of reduced total radiation interception, particularly in early deficit treatments. Both water use efficiency and transpiration efficiency increased with water deficit, even though soil evaporation as a proportion of total water use also increased with deficit. There was no stage of crop development at which yield was particularly sensitive to water deficit, although yield components changed with timing of deficit. Importantly, measures of potential soil moisture deficit integrated the effects of timing and severity of drought, making it possible to simply and mechanistically account for the effects of water deficit on biomass and particularly yield.

Shoot growth on de-fruited grapevines: a physiological indicator for irrigation scheduling

A simple and inexpensive but biologically meaningful way of gauging grapevine water status that can guide irrigation scheduling is described. The growth rate of shoots on grapevines from which all fruit has been removed (de-fruited shoot growth) is presented as a sensitive indicator of vine and soil water status and we propose a procedure for irrigation scheduling based on monitoring de-fruited shoot growth and soil moisture. These guidelines were derived from corresponding measurements of de-fruited shoot growth and soil moisture potential that were undertaken at frequent intervals (generally 1 to 2 days). De-fruited shoot growth rate between 48 and 71 days after anthesis was linearly correlated with the mean moisture potential of the soil at depths of 60 cm and 90 cm in the centre of the drip-irrigated zone (r = 0.94). De-fruited shoot growth effectively ceased when soil moisture potential at these points had decreased to -70 kPa, although much of the root zone would have been even drier. De-fruited shoot growth thus shows an integrated and dynamic response to vine-available soil water that could aid irrigation scheduling.

Spatial variability in soil nitrogen dynamics after prescribed burning in Ohio mixed-oak forests

This study describes the results of the application of a single dormant season prescribed fire to two southern Ohio forest sites for the purposes of restoring the ecosystem functional properties that existed in these sites prior to major human intervention (clearcutting, fire suppression, and atmospheric deposition). Each forest site was composed of three contiguous watershed units, two of which were burned in April of 1996. The forest sites differed in soil pH and available litter mass prior to the fires, and in both sites pH and available inorganic N varied among landscape positions such that inorganic C increased with increasing longterm soil moisture potential (measured as the GIS-derived Integrated Moisture Index [IMI] developed for this region). The fire temperatures at 10 cm above the litter surface were generally 150–300 °C, and 29–80% of the litter was consumed, depending on site and landscape position. Soil solution total inorganic N (TIN) present one month after the fires did not differ significantly from that present prior to the fires in either burned or unburned watersheds, but was consistently greater in mesic landscape positions than in more xeric ones. N mineralization potential and organic C content varied both among fires and landscape positions. At the site which burned at higher intensity, soil N mineralization and TIN were both decreased by fire. At the less intensely burned site, fire resulted in increased TIN in the soils from the more xeric landscape position, and greater soil organic C in soils from the intermediate soil moisture areas. Path analysis produced models for fire-induced changes in C and N dynamics capable of explaining 26–69% of the observed variation using combinations of landscape and fire behavior. Losses of N to volatilization from these single fires were generally < 1 kg N/ha, and thus could not be expected to ameliorate the effects of atmospheric N deposition in these sites.

Leaf injury characteristics of grassland species exposed to ozone in relation to soil moisture condition and vapour pressure deficit

A range of plant species typical of semi‐natural grasslands were tested for their sensitivity to short‐term ozone injury under normal and reduced irrigation, and in relationship to air vapour pressure deficit. Potted specimens of 24 herbs, legumes and grasses were exposed during two seasons to four O3 treatments in open‐top chambers. The ozone treatments were: (a) charcoal‐filtered air; (b) charcoal‐filtered air plus ozone to match ambient levels; (c) charcoal‐filtered air plus O3 to ambient levels 1.5 and (d) charcoal‐filtered air with ozone added to twice ambient levels during selected episodes of 7–13 d. During these ozone episodes, half of the plants in each ozone treatment received reduced irrigation (dry treatment) while the rest was kept under full irrigation (wet treatment). Type and date of first occurrence of leaf injury were noted during individual growth periods. Plants were harvested three times per year, and the percentage of injured leaves was recorded. Depending on species, injury symptoms were expressed as flecking (O3‐specific injury), leaf yellowing or anthocyanin formation. Carum carvi and most species of the Fabaceae family (Onobrychis sativa, Trifolium repens, Trifolium pratense) were found to be most responsive to O3, injury occurring after only a few days of exposure in treatment (b). An episodic reduction in irrigation tended to reduce the expression of O3‐specific symptoms, but only in species for which a reduction in soil moisture potential and an associated reduction in stomatal conductance during the dry episodes were observed. In other species, the protection from O3 injury seemed to be of little importance. Using artificial neural networks the injury response of nine species was analysed in relation to Species, stomatal conductance, ozone as AOT40 (accumulated exposure above a threshold of 0.04 ppm for periods with global radiation 50 W m−2 (Fuhrer et al., 1997)), mean relative growth rate, air vapour pressure deficit and global radiation. In the model with all factors, Species was most important, and when Species was omitted, stomatal conductance was the most important determinant for leaf injury to occur, whereas mean relative growth rate was less important. With no plant‐related factors included, air vapour pressure deficit and AOT40 were of highest importance. Only in eight species was a positive relationship found between these two factors during the five days before the onset of injury, indicating increasing protection from ozone with increasing air vapour pressure deficit in some but not all species. These data show that across a range of grassland species, leaf injury caused by elevated levels of ozone is most likely to occur in species with high stomatal conductance and that protection from ozone during dry periods is species‐specific and depends on a reduction in stomatal conductance due to a decrease in soil moisture potential. Protection under increased vapour pressure deficit can occur in some but not all species, depending on the relationship between stomatal conductance and air vapour pressure deficit.

Effect of Mulches on Persistence of Entomopathogenic Nematodes (Steinemema spp.) and Infection of Strobilomyia neanthracina (Diptera: Anthomyiidae) in Field Trials

The effect of mulches on the persistence of entomopathogenic nematodes, Steinernema spp., for infecting larvae of the spruce cone maggot, Strobilomyia neanthracina Michelsen, was tested in a series of field trials conducted in New Brunswick, Canada. Peat mulch increased the percentage of maggots infected with Steinemerna feltiae strain 27 (= bibionis ) (Filipjev) strain 27, S. feltiae Umea , and Steinernema carpocapsae (Weiser) All strain, when maggots were placed on nematode-treated soil beneath the peat layer. However, when maggots were placed on the surface of the peat (to simulate larvae dropping) in a subsequent experiment, the percentage of maggots infected by S. feltiae strain 27 was less than half of that for maggots placed beneath the peat. Significantly fewer maggots were infected in peat than in either sand or potting soil in a laboratory experiment. Mean percentage of maggots infected by S. feltiae strain 27 over a 21-d period after application was not increased by retaining a natural vegetative cover, or by mulching with hay, wood-bark chips, or bark. Bark mulch, however, did reduce the rate of decline in percentage of infection over time. The numbers of S. feltiae infective juveniles present in the soil from 0 to 3 wk after application were estimated in 2 field trials by repeated baiting with greater wax moth larvae. Numbers of infective juveniles per gram of soil were positively related to percentage of cone maggots infected in 1 of 2 yr, but were not significantly affected by mulching. Mulching with hay, bark, or wood-bark resulted in significantly lower soil moisture potentials compared with non mulched treatments. The results suggest that the mulches tested in this study would not maintain adequate efficacy or persistence of S.feltiae strain 27 for suppression of cone maggot over the 2- to 4-wk period of larval drop.

Towards the explanation of within-field variability of yield of winter barley: soil series differences

In a trial over three successive seasons, the yields of winter barley crops were mapped on a 6 ha field, the soils of which had been mapped according to a system of simple units corresponding to soil series as defined by the Soil Survey of England and Wales. Significant yield variation was explained by differences between soil map units, and the interaction with differences between seasons. The physical properties of the soil series and the potential soil moisture deficits in the three seasons suggested that moisture differences between the soils accounted for much of the observed differences in yield. The implications of these results for crop management in response to within-field variability are discussed.

Effects of Irrigation, Nitrogen Fertilization and Soil Type on Yield and Quality of Cauliflower

ABSTRACT The influence of irrigation, nitrogen rate and source, and soil type on yield, quality and nitrate nitrogen content of cauliflower was studied in field experiments. Irrigation provided at soil moisture potentials of -20 and -40 kPa secured a higher yield than that of -60 kPa. Yield of cauliflower curds increased with nitrogen rates up to 600 kg N/ha. Among the tested nitrogen fertilizers, calcium nitrate produced the highest yield of cauliflower curds. Out of five tested soil types, highest cauliflower yield was obtained on a low moor peat and on silty clay loam. Nitrogen fertilization resulted in a linear increase of nitrate nitrogen in cauliflower leaves and curds. Nitrate nitrogen content of leaves and curds was influenced by cultivar, soil type, and rate and form of nitrogen.

Comparison of soil phosphorus extractants as affected by fertilizer phosphorus sources, lime recommendation and time among four Arkansas soils

Phosphorus (P) sources were evaluated for their usefulness as P fertilizers in four acid Arkansas soils. Soils were amended with acidulated rock phosphorous, Florida rock phosphate, liquid ammonium phosphate, North Carolina rock phosphate, and triple superphosphorous, at 50 mg P kg‐1. After application, the soils were incubated for two years at 33 kPa soil moisture potential, with samples taken periodically. At each sampling Mehlich III (M‐III) and Bray‐Kurtz PI (BK1) extractants were used to measure available P levels. Acidulated rock phosphate was consistently the best P source for all soils, although P levels were significantly raised above the check by all P sources. Mehlich III extracted significantly more P than did BK1 for all soils. Phosphorus release varied among amendments during the incubation period. Acidulated rock phosphate, liquid ammonium phosphate, and triple superphosphate exhibited rapid initial release. The Florida and North Carolina rock phosphates did not achieve their maximum P release until one year of incubation. The correlation between extractants for P was M‐III = 1.2 + 1.06 (BK1) with r2 = 0.94.

RESPONSES OF JUVENILE TREES TO ABOVE- AND BELOWGROUND COMPETITION IN NUTRIENT-STARVED AMAZONIAN RAIN FOREST

In several tropical lowland rain forests, removal of root competition by trenching around seedlings and saplings has been shown to have little or no impact on growth rate. However, we found that trenching increased the aboveground growth rate of saplings and seedlings of a wide variety of species in an Amazonian caatinga. This low-statured forest develops on waterlogged, humus-rich, white-sand soil that is extremely low in available nitrogen. Analysis of foliar nutrient concentrations and a fertilizer experiment confirmed previous findings that nitrogen is limiting, while measures of soil moisture potential and stomatal resistance gave no indication of water shortage. We suggest that belowground competition has an impact on growth rate because (1) established trees allocate a large proportion of biomass to fine-root production, further reducing the availability of nutrients in an inherently poor soil, and (2) penetration of daylight to the sapling layer is greater than under other tropical lowland rain fo...

Nitrogen mineralisation rates from soil amended with dairy pond waste

An open incubation and leaching study was conducted under controlled temperature (25°C) and moisture conditions to measure the N mineralisation rate in soil amended with dairy pond sludge. The dairy pond sludge was applied at 3 different rates equivalent to 0, 200, and 400 kg N/ha. The incubation was conducted at 3 different soil moisture potentials (0, -3, and -13 kPa). Following each 2-week period of incubation, the soil was leached with 2 pore volumes of deionised water to remove the mineralisation products. Mineralisation products in the leachate and enzyme activities, microbial biomass C and N, pH, and water-soluble C in the soil were determined. The incubation lasted 18 weeks. Rapid release of nitrate occurred during the first 6 weeks of incubation, followed by a slow release over the remainder of the incubation period. Although the total amount of N released in the 200 kg N/ha treatment (169 mg N/kg soil) was less than in the 400 kg N/ha treatment (206 mg N/kg soil), when expressed as a percentage of the organic N applied, the amount of N released at the lower rate (18·4%) was greater than that at the higher rate of sludge treatment (13·0%). Rapid nitrification decreased the soil leachate ammonium concentration and the soil pH. Soil microbial biomass, water-soluble C, and deaminase activity were significantly increased after the addition of dairy pond sludge. The increase in soil microbial biomass observed was probably due to the increased water-soluble C and nutrients that stimulated the soil microbial growth. The rapid N release and nitrification rates observed were attributed to the low C : N ratio (12·7), high ammonium content (145 mg N/kg) of the dairy pond sludge used, and the optimum moisture and temperature conditions. The narrow range of soil water potential conditions did not have any significant effect on N release rate or amount.

Recharge components in a semi-arid area: the Sana’a Basin, Yemen

Abstract Credible estimates of recharge are essential for assessment of sustainable water resources in semi-arid areas, which rely on groundwater for irrigation and domestic supply. Investigation of the mechanisms and rates of recharge to the aquifers of the Sana’a Basin, Yemen, showed that the main components of recharge are infiltration of surface flows, irrigation return and urban leakage. Limited data and inaccessible, mountainous terrain required the use of robust, low-cost methods of quantifying the recharge components. Infiltration of surface flows from 12 wadis averages 39 Mm 3 a −1 , with high interannual variation. The recharge was calculated using a combination of a rainfall-run-off model and computation of the water balance for the wadi. Irrigation return flows of 30 Mm 3 a −1 , 20 to 40% of the water applied, were derived from measurements of soil moisture and matric potential for the three most extensive crops. Urban recharge from mains leakage and seepage through septic tanks was estimated from a well inventory, water supply records and usage rates, as 59% of the water supplied. The total recharge to the aquifers of the Sana’a Basin in 1993 was estimated to be 102 Mm 3 , significantly less than the total groundwater abstraction, of about 184 Mm 3 .