Shallow Groundwater Table Research Articles

Uncertainties in leaching assessment in micro-irrigated fields using water balance approach

Leaching is an important aspect of irrigation water management, as it must be minimal to save available irrigation water resources, prevent shallow groundwater tables, and reduce nutrient loadings to the groundwater. However, at the same time, leaching should be sufficient to maintain root zone salinity levels below the threshold to prevent yield reduction. Therefore, monitoring leaching is the key component in evaluation and optimization of irrigation water management practices. Water balance (WB) is a common approach used to estimate leaching in agricultural fields and was applied in this study to assess field-scale leaching and the associated uncertainties for an almond orchard under drip and micro-sprinkler irrigation systems. In this study, we showed that change is soil water storage (ΔS) is highly influenced by the extent of monitoring depth, the location and number of monitoring points. Local measurement of WB parameters showed that leaching is highly variable across the field, thereby introducing considerable uncertainty on estimated leaching using WB approach. It was also shown that unknown input of water through fog interception added to the complexity of closing water balance at field scale.

Assessing the trade-off between shallow groundwater conservation and crop production under limited exploitation in a well-irrigated plain of the Haihe River basin using the SWAT model

Groundwater overdraft due to extensive irrigation has led to a shallow aquifer depletion crisis in the Haihe River basin. Quantitative simulation of the variations in shallow groundwater and crop yield under different conditions of limited exploitation using a distributed hydrological model is important in the well-irrigated plain of this basin. Based on multiple modeling experiments by a modified Soil and Water Assessment Tool (SWAT) model, three limited irrigation schemes for winter wheat (Triticum aestivum L.) were selected as simulation scenarios. The simulated results using the SWAT model showed that under scenario 1 (applying two rounds of irrigation corresponding to the jointing and heading stages of winter wheat), the average rate of decline in the shallow groundwater table was approximately 2/3 of that under the basic scenario (current irrigation schedule), but the winter wheat yield decreased by 13% compared with the basic scenario. Under scenario 2 (applying one round of irrigation at the jointing stage of winter wheat), the average rate of decline in the shallow groundwater table was approximately 1/4 of that under the basic scenario, but the reduction in the winter wheat yield compared with the basic scenario increased to 28%. The amount of overexploited shallow groundwater in the cropland area decreased from 17.5 × 108 m3 a−1 (under the basic scenario) to 11.0 × 108 m3 a−1 under scenario 1 and 4.5 × 108 m3 a−1 under scenario 2. Under scenario 3 (rain-fed conditions during the winter wheat season), the regional variation in the shallow groundwater table shifted to a recovery trend with an average rate of 0.22 m a−1, which was equivalent to restoring 3.5 × 108 m3 a−1 of shallow aquifer storage in the cropland area. However, the reduction in the winter wheat yield compared with the basic scenario reached 54% under the rain-fed scheme. Considering the trade-off between groundwater conservation and crop production under limited exploitation, linear programming was used to optimize the irrigation schedule at the subbasin scale. As a result, to satisfy the constraint of stopping groundwater drawdown, the average minimal reduction in the winter wheat yield would be 42% under the optimal irrigation schedule. By contrast, to satisfy the constraint of restricting the reduction in the crop yield to within the threshold required to maintain winter wheat self-sufficiency, the minimal rate of decline in the shallow groundwater table would be 0.26–0.52 m a−1 under the optimal irrigation schedule. In addition, the uncertainty in the simulated shallow groundwater variation in this study was acceptable, indicating that the above assessments could provide reasonable references for regional groundwater management.

Shallow groundwater effects on evaporation and soil temperature in two windblown sands (Eutric Cambisol and Chromic Luvisol) in South Africa

The aim of this study was to investigate the effect of shallow groundwater tables on soil surface evaporation and its effect on temperature distribution on two aeolian soils in South Africa, Clovelly (Eutric Cambisol) and Hutton (Chromic Luvisol) soil forms. The study was conducted at a lysimeter under semi-arid climatic conditions. A constant groundwater tables were created at different depths (0 mm, 500 mm, 1000 mm, 1500 mm and a No WT (no water table treatment as a control) and each of the lysimeters were equipped with a 1200 mm long DFM soil moisture probe to measure the profile water content and soil temperature at 6 depths along the profile (every 200 mm depth). The research site was also equipped with a movable shelter to prevent rain water dilution during rainfall events. Finally, evaporation is calculated by the water balance approach. The ANOVA showed that there was a highly significant difference (P < .01) in rate of evaporation from the water table depths and soil types. The minimum and maximum evaporation were recorded in the deepest and shallowest water table depths. The diurnal soil water content shows that the maximum water content in the two soil profiles was recorded in the early morning times (6:00) and the minimum was in the late afternoon times (between 16:00–17:00). The diurnal variation of soil water content also increased with decreasing water table depth. Regarding the soil temperature, the ANOVA showed that the soil temperature was influenced by the interaction between water table and measurement depths. Hence, the top five minimum soil temperatures were recorded in the shallower (≤500 mm) water tables and the top 5 maximums were recorded in the deeper water table depths (1000, 1500 and No WT). Generally, as the groundwater table depth increased from the surface down the profile, the rate of evaporation decreased resulting an increase in the variation of temperature with in the diurnal cycle.

Incorporating Influences of Shallow Groundwater Conditions in Curve Number-Based Runoff Estimation Methods

Runoff generation process in any watershed is mainly affected by precipitation, land use and land cover, existing soil moisture conditions and losses. Shallow groundwater table conditions that occur in many regions are known to affect the soil moisture retention capacity, infiltration and ultimately the runoff. A methodology that links soil moisture capacity to the shallow groundwater table or High-Water Table (HWT) using a nonlinear functional relationship within a curve number (CN)-based runoff estimation method, is proposed and investigated using single and continuous event simulation models in this study. The relationship is used to obtain an adjusted CN that incorporates the effect of change in soil moisture conditions due to HWT. The CN defined for average conditions is replaced by this adjusted CN and is used for runoff estimation. A single event model that uses Soil Conservation Service (SCS) CN approach is used for evaluation of variations in runoff depths and peak discharges based on different HWT conditions. A real-life case study from central Florida region in the USA was adopted for application and evaluation of the proposed methodology. Results from the case study application of the models indicate that HWT conditions significantly influence the magnitudes of peak discharge by as much as 43% and runoff depth by 48% as the water table height reaches the land surface. The magnitudes of increases in peak discharges are specific to case study region and are dependent on the functional form of the relationship linking HWT and soil storage capacity. Also, for specific values of HWT, an equivalency between HWT-based CN and wet antecedent moisture condition (AMC)-based CN can be established.

Review and Case Studies on Biodrainage: An Alternative Drainage System to Manage Waterlogging and Salinity

AbstractWaterlogging and its associated soil salinity are a major impediment to the sustainability of irrigated agriculture. Although conventional engineering drainage technologies such as subsurface or vertical drainage combat the problem, they are costly and generate huge quantities of effluent which is difficult to dispose of. Biodrainage, which removes excess soil water by deep‐rooted fast‐growing trees through evapotranspiration using bioenergy, is an appropriate alternative. Plant consumptive water use varies between 6500 and 28 000 m3 ha‐1 yr‐1 and an ideal tree plantation lowers groundwater tables by 1–2 m over a time period of 3–5 years. Trees such as Eucalyptus, Populus, Casuarina, Dalbergia, Syzigium, Acacia, Prosopis, Leucaena, etc. are found to be effective in lowering shallow groundwater tables and reversing salinity trends. Integration of trees such as Eucalyptus and Populus along with crops in a unified agroforestry system, on approach roads, field bunds or on dykes of ponds in an integrated farming system, is another viable proposition. Irrigation of highly transpiring trees has also been proposed for reuse of wastewater and conservation of nutrient energy into biomass, leading to carbon sequestration, environmental sanitation and eco‐restoration. For effective understanding and implementation, several case studies on the role of biodrainage for managing waterlogging and salinity are discussed in this paper. Copyright © 2018 John Wiley &amp; Sons, Ltd.

Effects of shallow groundwater table and fertilization level on soil physico-chemical properties, enzyme activities, and winter wheat yield

Waterlogging adversely affects winter wheat (Triticum aestivum L.) growth by deteriorating soil environmental factors. The objective of this study is to test the effects of groundwater depth and fertilization level on soil properties, enzyme activities, and wheat yield. Experiments were carried out in micro-lysimeters at the groundwater depths of 0.2, 0.4, 0.5, 0.6, and 0.8 m under low, normal, and high fertilization levels in the winter wheat growth season from Oct. 2015 to May 2016. Soil water content, pH, organic matter content, total N, available P, available K as well as sucrase, urease, and phosphatase activities were measured in various growth stages of winter wheat, and yield component and grain yield were also measured after its harvest. Results indicated that soil water content and pH decreased with the increased groundwater depth. The lower contents of soil organic matter and nutrients appeared when groundwater depth was 0.5–0.6 m. Sucrase in the heading stage, urease in the jointing stage, and phosphatase activities increased significantly with the increased groundwater depth. Grain yield increased with groundwater depth to its maximum or approximate steady value. Total N and available P contents and urease activity appeared to increase with fertilizer application rate but available K content showed a contrary tendency. Grain yield increased with fertilizer application rate, but the effect of fertilization level on grain yield was lower than that of groundwater depth. A significant interaction between groundwater depth and fertilization level on grain yield existed. Soil enzyme activities were significantly correlated with available P content, and phosphatase activity was correlated with soil water, pH, organic matter, total N, and available K contents. It can be concluded that controlling groundwater depth and appropriate fertilizer application level can improve crop growth environment and promote winter wheat growth and its grain yield.

Liquefaction severity mapping based on SPT data: a case study in Canakkale city (NW Turkey)

Soil liquefaction is one of the major causes of damage to buildings and structures during earthquakes. Very shallow groundwater table in Quaternary alluvial deposits and the seismic properties of a region can cause a significant damage to buildings and infrastructure dependent on liquefaction. Canakkale city is located in the first-degree seismic hazard zone according to the earthquake zone map of Turkey. A large part of the Canakkale settlement area is located on unconsolidated alluvium recently deposited by the Saricay River. In this survey, the liquefaction potential of the Canakkale settlement area was investigated based on the liquefaction severity index and liquefaction potential index for two possible earthquake scenarios with a moment magnitude (Mw) and peak ground acceleration (amax) of 7.5 and 319 gal and 7.0 and 222 gal, respectively. In addition, these two methods were analysed using the peak ground acceleration (amax = 141 gal) value measured at the Canakkale station during the 2014 Aegean Sea earthquake. Based on the results of the analyses, liquefaction susceptibility maps of Canakkale city were produced for different amax values. The study involved three stages: field work, laboratory testing, and generation of the liquefaction severity maps. Geotechnical boreholes at 151 locations were drilled and Standard Penetration Tests (SPT) performed. Thereafter, natural moisture content, unit weight, grain-size distribution, and Atterberg limits were determined by means of laboratory testing. Finally, Quaternary alluvial deposits in the study area were divided into five classes representing very low-to-very high liquefaction for three amax values.

Assessing the Influence of Precipitation on Shallow Groundwater Table Response Using a Combination of Singular Value Decomposition and Cross-Wavelet Approaches

Identifying the spatiotemporal change of the groundwater table to precipitation at the river basin scale can be important for regional water resource management. In this study, we proposed a method that combines singular value decomposition and cross-wavelet approaches to analyze the relationship between groundwater level dynamics and precipitation. The method was applied to the Naoli River Basin, Northeast China. Moreover, the method of continuous wavelet using fast Fourier transform was also used to reveal clearly the relationship between groundwater level and heavy precipitation. The results showed that the major mode of relationship between groundwater and precipitation was divided into four patterns in the study area. In general, the lag time is 27.4 (standard deviation: ±8.1) days in pattern 1, 107.5 (standard deviation: ±13.2) days in pattern 2, 139.9 (standard deviation: ±11.2) days in pattern 3, and 173.4 (standard deviation: ±20.3) days in pattern 4, respectively. In addition, the response of groundwater level dynamics is very sensitive to heavy precipitation in all patterns. Therefore, enhancing the utilization of heavy rainfall and flood resources is an effective way to increase groundwater recharge in this basin.

Liquefaction potential assessment of Guwahati city using first-order second-moment method

Soil liquefaction is the failure of the soil due to the sudden increase in a pore water pressure causing the effective stress to reduce significantly thus losing shear strength, with the resulting effect causing the fluid type behavior of the soil. Guwahati city, the capital of Assam, lies in the northeastern region of India and this whole region is considered seismically very active (seismically sixth position globally). The whole region is categorized in Zone-V according to the seismic zoning map of India as per IS 1893:2002 (Part 1). This region witnessed/experienced several major and great earthquakes in the past such as 1897 Shillong and 1950 Assam earthquakes. The soil deposits of the city mainly consist of an alluvial type of Holocene age and the presence of shallow groundwater table makes it vulnerable to soil liquefaction. In this study, an assessment of soil liquefaction potential of Guwahati city is performed based on the methods proposed by Youd and Idriss (J Geotech Geoenviron Eng 127:297, 2001) and Idriss and Boulanger (2010; CPT- and SPT-based liquefaction triggering procedures, 2014) using standard penetration test data. The evaluation of liquefaction potential is carried out for 82 borehole sites considering the Great 1897 Shillong earthquake of Mw 8.1 with a peak ground acceleration of 0.36 g. In the deterministic approach, the various parameters are involved in the evaluation of liquefaction potential, and the uncertainties of the input parameters as well as model cause the dissimilarity of the result. For instance, the same input parameters for both the models show different factor of safety. Hence, a comprehensive probability approach considering the uncertainty of parameters is essential for the evaluation of liquefaction susceptibility. In this study, reliability analysis for both the models based on first- order second-moment method has been used. The Reliability Index based on input parameters such as cyclic resistance ratio and cyclic stress ratio computed for both the models and subsequent liquefaction probability are established. The result shows the city is in most vulnerable condition even up to 15 m depth considering both the methods proposed by Youd and Idriss (J Geotech Geoenviron Eng 127:297, 2001) and Idriss and Boulanger (2010; CPT- and SPT-based liquefaction triggering procedures, 2014). Therefore, extra measure should be taken while constructing structure in and around the study area.

Seismic Liquefaction Potential in Muscat, Sultanate of Oman

Seismic liquefaction is a serious geotechnical engineering problem that takes place in saturated cohesion less soils during earthquakes due to the increase of pore-pressure so that the soil shear strength is decreased to zero. Muscat is situated in the north-eastern part of Oman close to Oman Mountains, which witnessed four earthquakes of order of 5.2 magnitude in the last 1300 years. The surface geology of Muscat reveals great variety of hard rocks in the eastern, southern and western parts to dense and lose sediments in the middle and northern parts. Muscat Municipality provided 1082 borehole data to be examined for their liquefaction susceptibility based on the soil characteristics. Susceptible soils only are further considered to liquefaction hazard assessment. Liquefaction occurs during an earthquake if the cyclic stress ratio (CSR) caused by the earthquake is higher than the cyclic resistance ratio (CRR) of the soil. CSR values were evaluated using probabilistic peak ground acceleration (PGA) values for return period of 2475 years at the surface given by detailed hazard and micro zonation studies. CRR for Muscat region is conducted based on the borehole data with N values of SPT tests, and shear wave velocity results from 99 MASW surveys over the entire region. All the required corrections to get standardized (N1) 60 values, to correct shear-wave velocity, and scale the results for Mw 6.0 instead of the proposed 7.5 are conducted. Liquefaction hazard maps were created using the minimum factor of safety (FS) at each site as a representative of the FS against liquefaction at that location. Results indicate that under the current level of seismic hazard, liquefaction potential is possible at some sites along the northern coast, where alluvial soils, shallow ground water table, and relatively high ground motion are present. The expected settlement of the soft soil at each liquefiable site is also evaluated.

Groundwater Vulnerability Mapping in Faisalabad District Using GIS Based Drastic Model

The demand of groundwater has significantly increased during the past two decades to meet the food and fiber requirements due to constrained surface irrigation water. The groundwater has a major share to grow agriculture in Punjab-Pakistan, which produces more than 90% gains of the country. Beside the importance of groundwater, a major concern in recent years is the contamination of groundwater. The consecutive monitoring and mapping of aquifer water quality in large areas is a hectic job and not even economically feasible. Thus groundwater vulnerability maps are becoming more in demands to represents regional aquifer contamination potential. The aim of this research work was to find out the groundwater vulnerability potential zones in Faisalabad and its nearby areas, which is the fastest growing city of Pakistan. For this purpose, GIS based DRASTIC model was used to measure vulnerability related to agriculture in Faisalabad district. The inputs of the model are based on seven different layers. The results of the model were compared with groundwater samples. It was found that the DRASTIC model accurately identified the vulnerability of Faisalabad District with prediction efficiency of about 73%. The results revealed that more than 30% of study area was under high vulnerability potential. The areas under high vulnerable pollution are located in middle and upper part of Faisalabad District. The high vulnerable pollution in surrounding of Salarwala, Chak Jhumra and Khurianwala cities was due to shallow groundwater table, high recharge, gradual slope, sandy aquifer media and soil media consist of medium sand. The high vulnerable pollution in surrounding of Dijkot city was due to shallow groundwater table, gradual slope, vadose zone consist of course sand and high hydraulic conductivity. It is therefore recommended that Rainwater harvesting and ASR (Aquifer Storage and Recovery Wells) should be adopted in medium and high vulnerability areas to reduce water salinity.

The Hupsel Brook Catchment: Insights from Five Decades of Lowland Observations

Core Ideas Meteorological and hydrological variables have been measured since 1964. Focus was on rainfall–runoff process, land–atmosphere interaction, solute transport. Long time series allowed studying effects of land use and water management changes. The 6.5‐km2 Hupsel Brook catchment has been used as an example rural lowland area to understand rainfall–runoff processes, land–atmosphere interactions, and solute transport and to investigate how they are affected by changes in land use and water management. Meteorological and hydrological variables have been measured nearly continuously since 1964, including the 1976 drought and 2010 flood. In addition, more than 2200 water quality samples have been analyzed since the 1980s, with dedicated field campaigns focused on soil physics, evapotranspiration, and rainfall measurement. Novel insights based on these observations include the conclusion that shallow groundwater tables result in a coupled saturated–unsaturated zone and sustain evaporation in dry periods. Partitioning of rainwater between various flow paths is storage driven and therefore catchment wetness determines, together with groundwater–surface water interaction, the response of runoff to rainfall as well as solute transport.

Status quo and present challenges of the sustainable use and management of water and land resources in Central Asian irrigation zones - The example of the Navoi region (Uzbekistan)

Soil salinization is a major problem in many arid and agriculturally used regions and Central Asia is a prime example for this. Intensive irrigation, saline water resources, shallow groundwater tables and high evapotranspiration rates lead to the enrichment of salts in the top soils and a reduced soil productivity in the Central Asian lowlands. This is aggravated by a strong socioeconomic growth which results in an increase in the water consumption and the intensification of the farming practices (e.g. fertilizer application). This article analyses these problems for the Navoi province in Uzbekistan, a region located in the lower reaches of the Zarafshan River in the Kyzyl Kum desert. Between 2000 and 2015 detailed data on the groundwater table, groundwater salinity and the soil salinity were collected, complemented by meteorological data, soil characteristics, water use, and fertilizer statistics. Based on these datasets the ameliorate soil conditions in the Navoi irrigation zone and their temporal dynamic were assessed.The results show that both the groundwater and soil salinity have improved slightly during the last 15 years. Especially the percentages of samples with a high salinity have been reduced significantly. In 2015 only 0.7% of the irrigated soils have been categorized as highly saline while 71.3% were only slightly saline and 18.3% non-saline, while those values for the year 2000 were 6.1%, 51.5%, and 13.2%. The ameliorate soil conditions, on the other hand, have not improved accordingly, but showed a slight deterioration, even if they are overall satisfactory in 79.6% of the area. The percentage of irrigated soils with an unsatisfactory condition has increased from 3 to 13.9% between 2000 and 2014. This development is not related to any climatological, soil or groundwater parameters, so that the impacts of an increased water usage for urban and industrial purposes (and the related effects on the water quality), as well as the increased fertilizer application, seem to be a possible explanation for the observed trend.The results show the impact of the low efficiency of the irrigation and drainage system, which is outdated, not well maintained and thus often failing, on the groundwater level and the soil salinization, but also how the attempts to update the system during the recent years has led to a slight improvement of the soil and groundwater salinity.

Streamflow and Sediment Yield Prediction for Watershed Prioritization in the Upper Blue Nile River Basin, Ethiopia

Inappropriate use of land and poor ecosystem management have accelerated land degradation and reduced the storage capacity of reservoirs. To mitigate the effect of the increased sediment yield, it is important to identify erosion-prone areas in a 287 km2 catchment in Ethiopia. The objectives of this study were to: (1) assess the spatial variability of sediment yield; (2) quantify the amount of sediment delivered into the reservoir; and (3) prioritize sub-catchments for watershed management using the Soil and Water Assessment Tool (SWAT). The SWAT model was calibrated and validated using SUFI-2, GLUE, ParaSol, and PSO SWAT-CUP optimization algorithms. For most of the SWAT-CUP simulations, the observed and simulated river discharge were not significantly different at the 95% level of confidence (95PPU), and sources of uncertainties were captured by bracketing more than 70% of the observed data. This catchment prioritization study indicated that more than 85% of the sediment was sourced from lowland areas (slope range: 0–8%) and the variation in sediment yield was more sensitive to the land use and soil type prevailing in the area regardless of the terrain slope. Contrary to the perception of the upland as an important source of sediment, the lowland in fact was the most important source of sediment and should be the focus area for improved land management practice to reduce sediment delivery into storage reservoirs. The research also showed that lowland erosion-prone areas are typified by extensive agriculture, which causes significant modification of the landscape. Tillage practice changes the infiltration and runoff characteristics of the land surface and interaction of shallow groundwater table and saturation excess runoff, which in turn affects the delivery of water and sediment to the reservoir and catchment evapotranspiration.

Geotechnical properties of Rasht clay and silt in Northern Iran

Owing to stratified profile of the sedimentary soils and shallow groundwater table in the city of Rasht, clay and silt soils possess different consistencies in various locations. Excessive consolidation settlement of the clay and silt governs geotechnical design of structures built in the city. This study aimed at characterising geotechnical properties of Rasht clay and silt from the upper 10 m. It comprised of constructing a database by collecting previous subsurface investigations at 21 project sites (including 70 boreholes), boring four test pits along with laboratory experiments and statistical analyses of the data. Properties investigated herein included bulk unit weight, specific gravity, natural water content, clay fraction, Atterberg limits and plasticity parameters, SPT N value, shear strength and stiffness and consolidation properties. Variation of these properties with depth was highlighted. Consistencies of the clay and silt specimens were determined. Laboratory strength and stiffness values were correlated to SPT N values. Rasht clay and silt were compared to other well-documented fine-grained deposits in literature. New relationships were established to estimate compression and recompression indices of the clay and silt from their index properties.

Measures to mitigate direct flood risks at riverbank filtration sites with a focus on India

The direct entry of flood water, resultant contamination and damage to groundwater and riverbank filtration (RBF) wells frequently cause the disruption of drinking water production in flood-prone areas worldwide, especially in India and other regions of South and South East Asia influenced by the monsoon. The aim of this paper is to identify critical flood-prone points in the present practice of well construction in India. The focus is on field investigations conducted on wells at the RBF sites in Uttarakhand and at some sites in Germany. Methods to determine whether the annular space of vertical wells and caissons of large-diameter wells are watertight were tested for their practical application in the field in India and Germany. The investigations showed that for a caisson well in poor sanitary condition, flood water can short circuit into the well within 20–45 min along preferential flow paths to a shallow groundwater table. In case of vertical well chambers in Germany, >300 L of water seeped into the well chamber during a 10-min observation period. The sealing of the annulus between the borehole wall and casing of vertical wells and watertightness measures to well chambers and their covers and well heads to prevent short circuiting are discussed in context to prevalent practice in India.

Impact of a shallow groundwater table on the global water cycle in the IPSL land–atmosphere coupled model

The main objective of the present work is to study the impacts of water table depth on the near surface climate and the physical mechanisms responsible for these impacts through the analysis of land–atmosphere coupled numerical simulations. The analysis is performed with the LMDZ (standard physics) and ORCHIDEE models, which are the atmosphere-land components of the Institut Pierre Simon Laplace (IPSL) Climate Model. The results of sensitivity experiments with groundwater tables (WT) prescribed at depths of 1 m (WTD1) and 2 m (WTD2) are compared to the results of a reference simulation with free drainage from an unsaturated 2 m soil (REF). The response of the atmosphere to the prescribed WT is mostly concentrated over land, and the largest differences in precipitation and evaporation are found between REF and WTD1. Saturating the bottom half of the soil in WTD1 induces a systematic increase of soil moisture across the continents. Evapotranspiration (ET) increases over water-limited regimes due to increased soil moisture, but it decreases over energy-limited regimes due to the decrease in downwelling radiation and the increase in cloud cover. The tropical (25°S–25°N) and mid-latitude areas (25°N–60°N and 25°S–60°S) are significantly impacted by the WT, showing a decrease in air temperature (−0.5 K over mid-latitudes and −1 K over tropics) and an increase in precipitation. The latter can be explained by more vigorous updrafts due to an increased meridional temperature gradient between the equator and higher latitudes, which transports more water vapour upward, causing a positive precipitation change in the ascending branch. Over the West African Monsoon and Australian Monsoon regions, the precipitation changes in both intensity (increases) and location (poleward). The more intense convection and the change of the large-scale dynamics are responsible for this change. Transition zones, such as the Mediterranean area and central North America, are also impacted, with strengthened convection resulting from increased ET.

Quantifying the distribution of tracer discharge from boreal catchments under transient flow using the kinematic pathway approach

AbstractThis focuses on solute discharge from boreal catchments with relatively shallow groundwater table and topography‐driven groundwater flow. We explore whether a simplified semianalytical approach can be used for predictive modeling of the statistical distribution of tracer discharge. The approach is referred to as the “kinematic pathways approach” (KPA). This approach uses hydrological and tracer inputs and topographical and hydrogeological information; the latter regards average aquifer depth to the less permeable bedrock. A characteristic velocity of water flow through the catchment is further obtained from the overall water balance in the catchment. For the waterborne tracer transport through the catchment, morphological dispersion is accounted for by topographical analysis of the distribution of pathway lengths to the catchment outlet. Macrodispersion is accounted for heuristically by assuming an effective Péclet number. Distribution of water travel times through the catchment reflect the dispersion on both levels and are derived in both a forward mode (transit time from input to outlet) and a backward mode (water age when arriving at outlet arrival). The forward distribution of water travel times is further used for the tracer discharge modeling by convolution. The approach is applied to modeling of a 23 year long chloride data series for a specific catchment Kringlan (Sweden), and for generic modeling to better understand the dependence of the tracer discharge distribution on different dispersion aspects. The KPA is found to provide reasonable estimates of tracer discharge distribution, and particularly of extreme values, depending on method for determining the pathway length distribution. As a possible alternative analytical model of tracer transport through a catchment, the reservoir approach generally results in large tracer dispersion. This implies that tracer discharge distributions obtained from a mixed reservoir approach and from KPA are only compatible under large dispersion conditions.

Global distribution of groundwater‐vegetation spatial covariation

AbstractGroundwater is an integral component of the water cycle, and it also influences the carbon cycle by supplying moisture to ecosystems. However, the extent and determinants of groundwater‐vegetation interactions are poorly understood at the global scale. Using several high‐resolution data products, we show that the spatial patterns of ecosystem gross primary productivity and groundwater table depth are correlated during at least one season in more than two thirds of the global vegetated area. Positive relationships, i.e., larger productivity under shallower groundwater table, predominate in moisture‐limited dry to mesic conditions with herbaceous and shrub vegetation. Negative relationships, i.e., larger productivity under deeper groundwater, predominate in humid climates with forests, possibly indicating a drawdown of groundwater table due to substantial ecosystem water use. Interestingly, these opposite groundwater‐vegetation interactions are primarily associated with differences in vegetation than with climate and surface characteristics. These findings put forth the first evidence, and a need for better representation, of extensive and non‐negligible groundwater‐vegetation interactions at the global scale.

A transient model for temperature prediction in a salt-gradient solar pond and the ground beneath it

Salt-gradient solar ponds are cost-effective long-term solar collectors that can store low-grade heat and deliver it continuously. For design and operation purposes, it is important to develop computational tools that can represent energy fluxes at the interface between the bottom of the pond and the ground beneath it. In this study, a robust one-dimensional transient model is developed to represent the thermal evolution of a salt-gradient solar pond and the ground that surrounds it. The model was evaluated under different contrasting scenarios: buried or unburied ponds, artificially or naturally heated ponds and for deep or shallow groundwater tables. Experimental data from an indoor laboratory-scale solar pond were used for the development, calibration and validation of the model. A good agreement between experimental and modeled results was observed, with a root mean square error (RMSE) of 1.21 °C and 1.54 °C for the upper and lower convective zones respectively, during a 28-days validation period. Further, the model was validated using experimental data from three outdoor salt-gradient solar ponds obtaining RMSEs that ranged between 1.5 and 6.5 °C. Results show that dividing the ground into multiple layers contributes to the robustness of the model, as it allows the representation of the ground heat storage.