Karen G. Villholth

The agricultural groundwater revolution: opportunities and threats to development

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Processes of colloid mobilization and transport in macroporous soil monoliths

Transport of pesticides, PAH and other hydrophobic or surface-complexing contaminants in soils may be enhanced by colloid-facilitated transport. A prerequisite for colloid-facilitated transport is the release and transport of colloids. The mechanisms for colloid mobilization and transport in a macroporous Alfisol have been evaluated by measuring the amount and type of colloids leached in two large soil monoliths during long duration simulated rain events. The soil was irrigated with water having a chemical composition close to natural rainwater and at intensities as expected under natural conditions. The results showed that the colloids were primarily mobilized and transported in the macropores and that the source of colloids was not exhausted for extended rainfall duration. The first flush of water mobilized loosely bound colloids that had a high organic content relative to the bulk soil. After the initial release, the high ionic strength in the percolating water limited the mobilization. For prolonged leaching, the diffusion of colloids from the macropore walls appeared to rate-limit the mobilization process. During the late leaching phase, the rate of colloid mobilization was positively correlated with flow velocity.

Impacts of the 2004 tsunami on groundwater resources in Sri Lanka

The 26 December 2004 tsunami caused widespread destruction and contamination of coastal aquifers across southern Asia. Seawater filled domestic open dug wells and also entered the aquifers via direct infiltration during the first flooding waves and later as ponded seawater infiltrated through the permeable sands that are typical of coastal aquifers. In Sri Lanka alone, it is estimated that over 40,000 drinking water wells were either destroyed or contaminated. From February through September 2005, a team of United States, Sri Lankan, and Danish water resource scientists and engineers surveyed the coastal groundwater resources of Sri Lanka to develop an understanding of the impacts of the tsunami and to provide recommendations for the future of coastal water resources in south Asia. In the tsunami-affected areas, seawater was found to have infiltrated and mixed with fresh groundwater lenses as indicated by the elevated groundwater salinity levels. Seawater infiltrated through the shallow vadose zone as well as entered aquifers directly through flooded open wells. Our preliminary transport analysis demonstrates that the intruded seawater has vertically mixed in the aquifers because of both forced and free convection. Widespread pumping of wells to remove seawater was effective in some areas, but overpumping has led to upconing of the saltwater interface and rising salinity. We estimate that groundwater recharge from several monsoon seasons will reduce salinity of many sandy Sri Lankan coastal aquifers. However, the continued sustainability of these small and fragile aquifers for potable water will be difficult because of the rapid growth of human activities that results in more intensive groundwater pumping and increased pollution. Long-term sustainability of coastal aquifers is also impacted by the decrease in sand replenishment of the beaches due to sand mining and erosion.

Integrated mapping of groundwater drought risk in the Southern African Development Community (SADC) region

Groundwater drought denotes the condition and hazard during a prolonged meteorological drought when groundwater resources decline and become unavailable or inaccessible for human use. Groundwater drought risk refers to the combined physical risk and human vulnerability associated with diminished groundwater availability and access during drought. An integrated management support tool, GRiMMS, is presented, for the mapping and assessment of relative groundwater drought risk in the Southern African Development Community (SADC) region. Based on composite mapping analysis of region-wide gridded relative indices of meteorological drought risk, hydrogeological drought proneness and human groundwater drought vulnerability, the mapping results highlight consistent areas across the region with highest groundwater drought risk and populations in the order of 39 million at risk of groundwater drought at present. Projective climate-model results suggest a potentially significant negative impact of climate change on groundwater drought risk. The tool provides a means for further attention to the key, but neglected, role of groundwater in drought management in Africa.RésuméL’assèchement d’une nappe fait apparaître les risques d’une sécheresse prolongée avec la diminution des ressources en eau souterraine qui deviennent indisponibles ou inaccessibles pour l’utilisation humaine Le risque sécheresse concerne le risque physique combiné à la vulnérabilité humaine, associé à une diminution de la disponibilité et de l’accès à l’eau de la nappe. Un outil intégré, GRIMMS, est présenté pour cartographier et évaluer le risque d’assèchement de nappe dans la région de la Southern African Development Community (SADC). Basée sur l’analyse d’une grille composite régionale d’indices relatifs du risque météorologique de sécheresse, prédisposition de la nappe à la sécheresse et vulnérabilité humaine à l’assèchement de la nappe, la cartographie met en lumière à travers toute la région les zones présentant de façon constante le risque le plus élevé d’assèchement de nappe et des populations de l’ordre de 39 millions d’habitants exposées actuellement au risque de son assèchement. Les projections d’un modèle climatique suggèrent un risque potentiel significatif de changement climatique et d’assèchement de nappe. L’outil est un moyen d’apporter une nouvelle attention au rôle clef, mais négligé, de l’eau de nappe dans la gestion de la sécheresse en Afrique.ResumenLa sequía del agua subterránea denota la condición y el peligro durante una prolongada sequía meteorológica cuando los recursos de agua subterránea disminuyen y se convierten en no disponibles o son inaccesible para el uso humano. El riesgo de sequía del agua subterránea se refiere al riesgo físico combinado con la vulnerabilidad humana relacionado con la disminución de la disponibilidad y acceso de agua subterránea durante la sequía. Se presenta una herramienta soporte de manejo integrado, GRiMMS, para el mapeo y la evaluación del riesgo relativo de sequía de agua subterránea en la región de la Comunidad Sudafricana para el Desarrollo (SADC). Sobre la base de un análisis de mapeos compuestos de una cuadrícula a escala regional de los índices relativos de riesgo de sequía meteorológica, de las tendencias hidrogeológicas a las sequías y de la vulnerabilidad humana a la sequía de agua subterránea, los resultados de los mapas resaltan áreas consistentes a través de toda la región con los más altos riesgos de sequía del agua subterránea y en poblaciones del orden de 39 millones en riesgo de sequía del agua subterránea en la actualidad. Los resultados de modelos climáticos proyectivos sugieren potencialmente un impacto negativo significativo del cambio climático sobre el riesgo de sequía del agua subterránea. La herramienta provee un medio para una ulterior atención del rol clave, aunque descuidado, del manejo del agua subterránea de la sequía en África.ResumoA seca das águas subterrâneas denota a condição e o risco durante uma seca meteorológica prolongada, quando os recursos de água subterrânea diminuem e se tornam indisponíveis ou inacessíveis ao uso humano. O risco de seca das águas subterrâneas refere-se à combinação do risco físico e do risco de vulnerabilidade antrópica associado à diminuição da disponibilidade de água subterrânea e ao seu acesso durante a seca. É apresentada uma ferramenta de apoio à gestão integrada, GRiMMS, para a cartografia e avaliação do risco relativo à seca das águas subterrâneas na região da Comunidade de Desenvolvimento da África Austral (SADC). Com base na análise de cartografia compósita de toda a região, dividida em quadrículas contendo os índices relativos de risco de seca meteorológica, propensão de seca hidrogeológica e vulnerabilidade humana à seca das águas subterrâneas, os resultados da cartografia permitem destacar, no presente, áreas consistentes com maior risco de seca das águas subterrâneas em toda a região e que as populações em risco por seca das águas subterrâneas são da ordem dos 39 milhões. Os resultados de modelos de previsão climática sugerem um impacte potencialmente negativo significativo resultante das alterações climáticas no risco de seca das águas subterrâneas. A ferramenta fornece um meio para uma maior atenção para o papel chave, mas negligenciado, das águas subterrâneas na gestão da seca em África.

Groundwater irrigation for smallholders in Sub-Saharan Africa – a synthesis of current knowledge to guide sustainable outcomes

Groundwater irrigation for smallholder farmers in Sub-Saharan Africa is growing in extent and importance. This growth is primarily driven spontaneously by the farmers themselves, spurred by improved access to low-cost technologies for pumps and drilling services as well as market opportunities for produce. This paper presents a review of the current status and knowledge of the prospects and constraints for sustainable and pro-poor groundwater irrigation in Sub-Saharan Africa. Further unlocking the potential of groundwater irrigation for smallholders will require better integrated approaches, simultaneously addressing groundwater-access constraints as well as enabling factors.

Smallholder groundwater irrigation in Sub-Saharan Africa: country-level estimates of development potential

The abundance of groundwater resources of Sub-Saharan Africa is generally well recognized, but quantitative estimates of their potential for irrigation development are lacking. This study derives estimates using a simple and generic water balance approach and data from secondary sources for 13 countries. Even with conservative assumptions and accounting for water demands from other sectors, including the environment, a 120-fold increase (by 13.5 million hectares) in the area under groundwater irrigation is possible for the countries considered. This expansion could improve the livelihoods of approximately 40% of the present-day rural population.

Groundwater use in a global perspective: can it be managed?

In Giordano, Mark; Villholth (Eds.). The agricultural groundwater revolution: opportunities and threats to development. Wallingford, UK: CABIComprehensive Assessment of Water Management in Agriculture Series 3

The Effects of the 2004 Tsunami on a Coastal Aquifer in Sri Lanka

On December 26, 2004, the earthquake off the southern coast of Sumatra in the Indian Ocean generated far-reaching tsunami waves, resulting in severe disruption of the coastal aquifers in many countries of the region. The objective of this study was to examine the impact of the tsunami on groundwater in coastal areas. Field investigations on the east coast of Sri Lanka were carried out along a transect located perpendicular to the coastline on a 2.4 km wide sand stretch bounded by the sea and a lagoon. Measurements of groundwater table elevation and electrical conductivity (EC) of the groundwater were carried out monthly from October 2005 to August 2007. The aquifer system and tsunami saltwater intrusion were modeled using the variable-density flow and solute transport code HST3D to understand the tsunami plume behavior and estimate the aquifer recovery time. EC values reduced as a result of the monsoonal rainfall following the tsunami with a decline in reduction rate during the dry season. The upper part of the saturated zone (down to 2.5 m) returned to freshwater conditions (EC < 1000 µS/cm) 1 to 1.5 years after the tsunami, according to field observations. On the basis of model simulations, it may take more than 15 years for the entire aquifer (down to 28 m) to recover completely, although the top 6 m of the aquifer may become fresh in about 5 years.

The agricultural groundwater revolution: setting the stage

In Giordano, Mark; Villholth (Eds.). The agricultural groundwater revolution: opportunities and threats to development. Wallingford, UK: CABIComprehensive Assessment of Water Management in Agriculture Series 3

Identifying the barriers and pathways forward for expanding the use of groundwater for irrigation in Sub-Saharan Africa

Sub-Saharan Africa (SSA) must significantly increase the area under irrigation for a host of pressing reasons that include addressing rural poverty, improving food security and countering droughts and famines. What remains less well known is the role that groundwater will play within the portfolio of potential irrigation strategies. Surface-water systems requiring large-scale public investments, which have captured most of the attention to date, do not always perform adequately, and are ill suited to targeting widely dispersed smallholder farming communities. Groundwater reserves are thought to be vast and broadly accessible, but estimates of their potential are still uncertain and the best development strategies are not always clear. Major challenges emerge on a number of fronts, notably inadequate knowledge of aquifer systems, the costs of drilling wells and operating pumps, and the level of technical and institutional capacity and overall governance in place to support sustainable management. This special issue of Water International (along with a supplementary section in a subsequent issue this year) brings together the key findings of two major research efforts supported by the Rockefeller Foundation and the Bill & Melinda Gates Foundation and led by the International Water Management Institute (IWMI) with the support of numerous participating partners, many of whom are featured herein. The individual papers cover a number of topics, including: