(pdf in Spanish) Policy synthesis report led by Dr. Susana del Granado, and published by Instituto de Estudios Avanzados en Desarrollo (INESAD) in Bolivia. This report resulted from the interviews that were conducted in Ecuador, Bolivia and the Dominican Republic as part of the IAI seed grant project on local flood response capacities. We hope that this will serve as a useful contribution to guide science-policy and climate risk management in the region.
Shallow groundwater and agricultural land management in the Pampas
This figure shows annual transpiration rates in the Argentinean Pampas (circles and lines) under two land uses: perennial alfalfa pasture, widespread until the early nineties, and annual wheat-soybean-maize rotations, the dominant land use today. Transpiration rates (right-hand axis) are higher under pastures than under annual crops (1200 vs. 800 mm/year, on average). A 100-year simulation of the depth to the ground water-table under these regimes of evapotranspiration is shown by the bars whose height indicates how often in these 100 years a given groundwater level is reached (frequency on the left-hand axis). The model shows that pastures maintain ground water tables more commonly at greater depths (average -4.4 vs. -2 m) and result in less frequent flooding (2 vs. 47 events).
Science Snapshots 7 – The Story
The flat sedimentary soils of the Pampas store rainfall surplus of wet years as groundwater. If such surpluses increase, the groundwater table can become so shallow that the land floods. Cyclic floods have taken several years to develop and retreat. The most recent 1997-2003 flood covered 27% of the western Pampas as average groundwater levels rose 2.3 meters, causing great economic loss. This cyclic flooding, which affects 16 million hectares of land, is often attributed to rainfall variations, but a team led by Esteban Jobbágy at the Faculty of Agronomy, University of Buenos Aires, found that land use plays a critical role. Different plants use different amounts of water, and under annual crop production, fields remain bare without any plant water use for part of the year. Deep-rooted, year-round alfalfa keeps soils drier while rotating grain crops let surplus water accumulate. Each crop also has an optimum range of groundwater levels at which it grows best. Outside that range, stress from either drought or water-logging will reduce productivity. As stress from water logging shuts plants down, water use is reduced and flooding becomes even more likely.
In the early nineties, it became more profitable to grow grains than to produce meat. Widespread soybean-wheat-corn rotation, grown year after year, increased flooding frequency and reduced both the number of years and the area suitable for farming. The team therefore explored land use alternatives within a grain production system using a simple simulation model, field observations of groundwater use and depth, and interviews with farmers.
Frequency of waterlogging can be reduced and the number of years suitable for farming increased by 40% by planting pastures or cover crops (alfalfa, ryegrass) tolerant to water-logging during periods of high flood risk. These crops are productive and their transpiration helps keeping the water tables down.
A new, flexible groundwater-based decision making strategy is now being adopted by farmers, accompanied by a collaborative web-based groundwater monitoring initiative (link below). As part of the partnership developed with scientists, farmers provide detailed yield maps and observations of water table.
Aragón R., EG Jobbágy, E. Viglizzo (2010): Surface and groundwater dynamics in the sedimentary plains of the Western Pampas (Argentina). Ecohydrology, online, DOI: 10.1002/eco.149
Nosetto M.D., E.G. Jobbágy, G.A. Sznaider, R.B. Jackson (2009): Reciprocal influence between crops and shallow ground water in sandy landscapes of the Inland Pampas. Field Crops Research, 113: 138-148
Jobbágy E.G., M.D. Nosetto (2008): Como hacer de las napas un aliado. Pautas y criterios para el monitoreo de niveles freáticos en sistemas de producción agrícola en la región pampeana. Revista CREA 328:32-38
Project web page
Collaborative website maintained by INTA, Universidad Nacional de San Luis, IyDA (a private company), and farmers.
Download PDF in Spanish
Potentially deadly errors in hurricane track prediction
This map shows the Eastern Pacific and Mexico with the Baja California peninsula. The colored lines correspond to the different official forecasts for the track of Hurricane John. John passed through the region between August 30 and September 4, 2006. The forecasts were issued by the National Hurricane Center (NHC) 72 hours (yellow line), 48 hours (red), and 24 hours (green) before landfall. All these forecasts indicated that John would pass out into the Pacific. But, as the blue line of John’s actual trajectory shows, this hurricane made landfall at the Southern tip of Baja California, and then moved through the length of the peninsula, where it produced heavy rains leading to flash-floods and five deaths in several mountain villages, and affecting the State capital La Paz.
The green dots in the graph represent upper air climate monitoring stations and the numbers correspond to upper air soundings with weather balloons in a three-day period at 00 hrs (first digit) and 12 UTC (second digit). Upper air soundings provide crucial data for the numerical models that are used to make the hurricane track predictions. Note that most stations indicate that no balloons were launched during John’s approach.
Science Snapshots 1 -The Story
The eastern Pacific basin has the largest density of tropical cyclones in the world, yet remains one of the least studied regions. Upon landfall, tropical cyclones may produce heavy downpours, causing landslides and flooding. Mountain ranges run parallel to the coast, both on the continent and the Baja California peninsula, increasing both rain intensities and the incidence of floods and landslides. The Mexican Meteorological Service and the US National Hurricane Center (NHC) issue forecasts of track and intensity for tropical cyclones, which are important for emergency managers to prepare defenses and warn the public. But, as the example shows, these forecasts are not always reliable enough.
To help improve the forecasting, Dr. Graciela Binimelis de Raga and her team at the Universidad Nacional Autónoma de México (UNAM), Centro de Ciencias de la Atmósfera, examined hurricane John in detail. John’s path was fairly typical for mid-summer cyclones, originating in the Gulf of Tehuantepec. Atmospheric conditions intensified the storm, and its passage over an ocean area with elevated heat content increased John to its maximum intensity.
The cyclone forecast issued at 9am on 30th August (yellow line in Figure) predicted no landfall at all. Later forecasts did predict landfall, but only brief contact some 50 km West from the actual landfall site. Only 47 hours before actual landfall, the model predicted an incursion onto land but its track was misplaced, predicting only limited impact from this storm. No predictions showed the actual path moving through the length of the peninsula, which is what brought death and destruction to large swathes of Baja California. In addition, satellite images predicted only moderate amounts (50-150 mm) of rainfall, whereas some weather stations recorded more than 440 mm of rain that resulted in heavy flooding and landslides. The science team identified a lack of airborne atmospheric measurements and a divergence of results between different models as the causes of the predictive failure.