INCREASING frequencies of droughts, coupled with increasing populations are requiring more water for irrigated agriculture.  

With the global population approaching nine billion by 2050, even more water will be required to produce an estimated 60 to 70 percent more food.  

These greater quantities of food production will require, at current water use efficiency rates, 50 percent more water.  

Consequently, the growing demand for food and fibre, combined with dwindling water supplies available, in terms of quantity and quality, for agricultural irrigation, require new soil technologies that conserve water. 

The Ogallala Aquifer in the US supplies 30 percent of groundwater for irrigated agriculture, and water supplies are being rapidly exhausted, especially in the southern regions. 

More frequent droughts with longer-term severity cause uncertainty in commodity prices and global food supplies.

During the last century, freshwater withdrawal increased globally over six-fold, from 579 to 3 750 cubic metres/year, due to increasing industrialisation, human and animal populations ans well as irrigated agriculture.  

The agricultural sector consumes approximately 70 percent of all freshwater, more than twice the amount of industrial, municipal and other users. 

According to the Organisation for Economic Growth and Development (OECD), much of the increase in agricultural water use has occurred in Australia, Greece, Portugal and Turkey, where agriculture’s share of total water use has exceeded 75 percent.  

In some countries, agricultural use has approached 98 percent.  

The annual unsustainable depletion of aquifers has surpassed 163,6 billion cubic metres/year. 

Approximately 75 percent of all irrigated land is located in developed countries where agriculture uses 73-88 percent of freshwater supplies. 

According to FAO Aquastat, Zimbabwe’s irrigation water requirement (all water sources), in1999, was 836 million cubic metres/year with 20 000 hectares equipped for irrigation by groundwater.  

In 2002, Zimbabwe’s fresh groundwater withdrawal for primary and secondary use was 420 million cubic metres/year.  

In 2007, freshwater withdrawal, as percentage of total renewable water resources, stood at 17,85 percent.  Industrial water withdrawal (all water sources) was 215 million cubic metres/year, while municipal water withdrawal (all water sources) was 425 million cubic metres/year and agricultural water withdrawal (all water sources) was 2930 million cubic metres/year.  

In 2014, the total internal renewable water resources in Zimbabwe stood at 6 000 cubic metres/inhabitant/year while groundwater produced internally was 6 000 million cubic metres/year. 

Ground-water is used to supply some urban water supply systems.

With relatively low rainfall and unequally distributed surface water resources, groundwater is the main water source in rural areas.  

It is also used for irrigation on some commercial as well as smallholder farms.  

However, despite its widespread use, groundwater was estimated to contribute not more than 10 percent of total water use in Zimbabwe.

The surprising factor of high water consumption is that, approximately 60 percent of all supplemental irrigation water is reported to be wasted.  

Thus, water and the reduction of deep percolation losses of root zone soil is becoming a major research focus among agricultural and hydropedological scientists and engineers 

According to the ‘Journal of Soil and Water Conservation’, a new soil water-saving technology is the innovative options for increasing food crops and fibre production with less water.

Sub-surface water retention has been practised for thousands of years by burying leaky clay pots in arid and semi-arid soils of Northern Africa and Iran.  

These same manually buried irrigation systems continue to be used in some arid lands in Africa, Asia and Latin America.  

They are reported to reduce soil salinity, increase irrigation efficiency, decrease irrigation frequency, improve crop yield and reduce labour.  

However, the pots experience microbial plugging that lowers wall permeability, resulting in lower plant productivity.

Alternatively, physical water barriers, mimicking natural thin clay layers, have been placed at certain depths in permeable soils.  

Spatially distributed clay layers with low permeability within some highly permeable soils increase the retention of both soil water and nutrients.

Clay mixed into surface sand soils and incorporations of thin layers of clay water barriers located 15-30cm beneath sandy soil surfaces provide modest improvements in soil-water retention crop yields as was often practised traditionally in Zimbabwe.  

However, applications of clay and silt materials in sandy soils are costly and create heterogeneous soils.  Recently, manual installations of polyethylene (PE) sheets have successfully increased plant yields in sandy biomass yields 1,56-fold with asphalt barriers than control models without barriers.

Although asphalt barrier retained more nitrogen (N) within the soil profile above the barriers, sugarcane yield was lower when grown on sands with shallow asphalt barriers due to oxygen deficiency and N volatilisation.

Sugarcane grown on asphalt barriers at 75cm depth required only 32 percent of the supplemental irrigation required by controls.  

Asphalt water retention barriers saved 841 900 l /ha of irrigation water annually, producing 92 percent more sugarcane biomass and 76 percent more sugar than controls in subtropical Taiwan.  

Asphalt water barriers installed at 75cm depths improved irrigation water use efficiency 333 percent and improved shoot-to-root ratios by 241 percent.  Asphalt barriers in sands established paddy rice (oryza sativa l.) produced an average of 208 percent more grain for spring and summer crops while requiring only 20 percent of supplementary irrigation water required by controls 

Mechanical installations of a thin layer of hot asphalt applied below the root zone in sand soils in Western Australia also doubled soil-water holding capacities, improved kiwi tree (actinidia deliciosa) growth, exhibited less nutrient stresses and produced more kiwi fruit than controls. 

In South Africa, asphalt barriers placed 60cm below the soil surface increased cotton (gossypium hirsutum l.) and alfalfa (medicago sativa l.) production by 25 percent and 30 percent respectively.  

Sugarcane biomass increased 1,5-fold when grown on dryland Fernwood sands.  

In India, there were reports of 70 percent less irrigation water required to increase yields of rice grown on asphalt barrier-improved sandy loam soils.  

Vegetable yields increased by 46 percent to 58 percent on bitumen barrier compared to control sandy loam soil. 

Scientists installed large PE sheets at 25 and 40cm depths to increase rice yields in north-east Thailand and Philippines on loamy sand to clay soils. 

Rice yields were zero without barriers in Thailand.  Rice yields grown on polymer lined sands ranged from 3,2 to 5,1 t/ha with water barriers and 1,5 to 3,3 t/ha without barriers in the Philippines.  

Studies of soil-water moisture profile in wheat (triticum) production on sand soils also reported that soils with subsurface barriers have greater increases in organic matter and plant nutrients.  

They also suggested higher quantities of sand fractions were retained in the cultivated depths of 0-30cm when barriers were present, compared to plots without the barriers.  

The greater retention of fine mineral fractions suggests Soil Water Retention Technology (SWRT) polymer membranes may also accumulate clay layers which retain more water in plant root zones.

In addition to field crops, subsurface polymer barriers have also been used to improve turf grass cultivation in China.  

Water savings of 35-70 percent were reported compared to control sands without water-saving membranes, at the same time producing high quality turf grass and 39 percent more turf grass clippings when water impermeable polymer layers were installed at 30 and 40cm to maintain water contents at 66 percent capacity in the sandy soils. 

Dr Tony M. Monda BSc, DVM, is currently conducting Veterinary epidemiology, agronomy and food security and agro-economic research in Zimbabwe. 

For views and comments, email: tonym.MONDA@gmail.com