Rehabilitation and Management Strategies of Overpumped Aquifers in a Changing Climate
Overpumping of aquifers due to agricultural irrigation is a global phenomenon. Among other undesirable consequences, overpumping leads to storage loss, diminishing an aquifer’s capability of buffering climatic extremes, which are increasing with climate change. This project offers a management system combining real-time monitoring, modeling and control to ensure sustainable use of a groundwater resource. The system was implemented in Guantao County in the North China Plain and serves as a blue print for affected regions elsewhere.
The North China Plain (NCP) is a global hotspot of groundwater overpumping. Being the breadbasket of China, its typical cropping structure is double cropping of winter wheat and summer maize. The overpumping, which is mainly caused by the winter crop, is in the order of 15 % to 20 % above average recharge. Showcasing a path to sustainable use of groundwater resources in such a hotspot can benefit global knowledge of sustainable groundwater management. The project pilot site, Guantao County of Handan Prefecture in Hebei Province, is a typical agriculturally dominated county in the NCP. It has an irrigated area of about 300 square kilometers. To get into an equilibrium of recharge and discharge, groundwater abstraction in the county has to be reduced by about 20 %.
Management System Design
To reach the goal, a management system has been set up, offering real-time monitoring, modeling and decision support for policy implementation. Monitoring data are transmitted to the decision support module, which identifies policy options. The chosen options are implemented in the field and the impact can in turn be assessed via the monitoring module. The cycle is repeated annually.
Surface water irrigation is easily monitored and controlled. Monitoring and metering pumping from millions of primitive irrigation wells is a more difficult task. Guantao County has about 8'000 pumping wells. Due to the common experience that mechanical meters are easily tampered with and even removed, we experimented with swipe card operated smart water meters with wireless data transmission, a system which has been applied successfully in other regions in China. The system turned out to be too expensive in maintenance in this rural area as meters broke or were sabotaged by the farmers. Moreover, the majority of wells is so primitive that the installation of a metering facility is virtually impossible. As, however, all wells are pumped with electric energy and each well has an electricity meter, we decided to use electricity consumed by a well as a proxy for the groundwater volume pumped. The conversion factor of a well is determined by a pumping test. The approach is scalable to the whole of the North China Plain and Hebei province has already adopted the method.
Groundwater levels are monitored by automatic observation wells, while land use and cropping area are monitored with satellite remote sensing. hydrosolutions GmbH has developed a Google Earth Engine web application to classify winter and summer cropping areas, providing annual cropping maps for typical crops such as winter wheat, summer maize and green houses in Guantao County and all of Handan prefecture.
Modeling and Decision Support
Water balances are computed and suggestions are made on the three available overpumping control measures: fallowing area of the winter crop, import of surface water and water saving. A water balance of the county for 2019 shows that the pre-proiect agricultural water gap of about 20 million cubic meters per year has been halved. The shallow aquifer is now more or less in equilibrium while for the deep aquifer, a deficit of roughly 10 million cubic meters per year still remains. This gap should be filled by targeting a combination of increased surface water import and increased winter crop fallowing to areas which for salinity issues of the shallow aquifer rely on deep water pumping. A distributed groundwater model shows the spatial variation of groundwater levels and the location of the depression cone. The deepest levels correspond to a district with a large green house area. Green houses have an even higher specific irrigation water demand per year than the winter wheat - summer maize crop rotation.
All modelling tools are integrated in an online decision support system, which provides help to local water managers regarding quota planning, crop water demand estimates, and groundwater level prediction using real time data and model results. The three tools, including the box model, a data driven groundwater level prediction model, and the irrigation calculator can easily be upscaled.
Three control measures were implemented in Guantao: these are subsidized fallowing of winter wheat, subsidies for water saving equipment, and setting quota for groundwater use. Water fees will be collected in the case that quota are overstepped.
Apart from fallowing, the remaining water saving potential for grain crops is generally low, as farmers already practice deficit irrigation. Low-till maize planting and plastic mulching are already widely used. So there is not much water saving potential left without sacrificing crop production, unless new water saving grain varieties can be deployed. Water saving by subsidized drip irrigation equipment for greenhouses has better prospects, given the associated savings in labor cost. Center-pivots as shown in the picture are not suited for the present smallholder farms. Water saving may become more effective and cost-efficient with the trend of aggregating small farms to bigger, industrial farms, which can practice precision agriculture.
Public Awareness for Sustainable Groundwater Use
The project team developed a groundwater game, starting from a board game, to a computer app. You can try out the game online on computer, the mobile phone or a tablet. The game was used to educate farmers about sustainability of groundwater use and to test farmers’ feedback to different policies. Together with CCAP’s team at Peking University we played the game app with farmers in the field.
Farmer players’ decisions in the game show strong similarity with their practices in reality: Farmer players show strong inertia in their crop choices, with their crop choice “this year” strongly depending on “last year’s” crop choice. This indicates that their crop preference is not mainly motivated by economic returns. It also indicates that once they have started winter wheat fallowing they may continue to plant only a single crop. This is consistent with the farmer household survey result that 43% of farmers claim that they will not replant winter wheat even after the fallowing subsidy stops. The satellite remote sensing results also show that the fallowing area of winter wheat in Guantao is now twice as large as the subsidized fallowing area of 20’000 Mu (about 1'300 ha) per year.
When playing the groundwater game, the groundwater level plays an important role in farmers’ water saving behavior, crop choice and adoption of water saving equipment. Giving this information to farmers in reality could facilitate farmers’ attitude towards sustainable groundwater usage.
Through project implementation, groundwater levels in Guantao’s shallow aquifer are more or less in balance. The continued decline of groundwater levels in deep aquifer can be stopped by a combination of increased import of surface water and further reduction of winter wheat production. The deep aquifer overpumping problem indicates that future water problems of agriculture in the region maybe more related to groundwater quality than quantity.
Subsidized fallowing has been an effective control measure. However, the amount of allowing cannot be increased substantially beyond the present level as it would contradict the food security policy of China.
Electricity to pumping volume conversion has proven to be an effective and feasible method for metering pumping wells, which can be upscaled to the whole of North China Plain.
Present water saving potential is low as farmers already save water through deficit irrigation. It will increase somewhat as small family farms are merged into large farms, which can practice precision agriculture.
The final solution of overpumping will come with the prolongation of the central route of the South North Water Transfer scheme into Hebei Province.
Capacity Building and Knowledge Sharing
Capacity building has been organized through project implementation, through field visits, annual workshops, study visits and work exchanges. During Covid-19, two training webinars on “Innovative technologies in groundwater modelling: Case study Guantao” and “Sino-Swiss cooperation on sustainable groundwater management” have been organized together with Chinese partners.
To share the knowledge we have learned from China globally, a set of E-Lectures was developed and has been made available to the global community of interested stakeholders. More information can be found here.