Understanding inflow water, soil water storage

 By Tai Kui

The issue of soil-water extraction and utilisation by plants is based on the recognition that the field, with all its components – soil, plant, and ambient atmosphere taken together – constitutes a physically integrated and dynamic system in which the various flow processes occur interdependently in a link called the soil-plant-atmosphere continuum (SPAC). Inflow water within this system in PNG agriculture normally comes from the atmosphere (rainfall) with few from irrigation and capillary rise (from sub soil water table). 

The soil’s ability to store this water for plant uptake depends on the soil texture (fineness and coarseness), structure (arrangement of different soil textures), organic matter content (decomposed materials), rainfall intensities, slope, vegetative cover and soil water potential (work done to transport very little amount of water to a low concentration area from a pool of pure water) etc.

Thus it creates demand for crop water need (crop water requirement (ETc)). Crop at each growth stages (initial, mid and end stage) need certain amount of water and that water mainly supplied by the inflow water and residual soil water.

Water demanded by the major tuber crop at different growth stages under normal scenario at specific agroecological zone is shown in figure 1 above. The optimum inflow water (rainfall) for most crops in PNG ranges between 1500 and 3000 mm/year. Most parts of PNG receive between 2000 and 3500 mm of rain per year. The drier areas such as Port Moresby, Markham in Morobe Bena Bena in Eastern Highlands receive 1000-2000 mm/year, while the wettest parts can receive as much as 8000-10,000 mm/year. PNG has a wet season from November to April and a dry season from May to October but these seasons are only noticeably different in Port Moresby, where about 78 per cent of the yearly average rainfall comes in the wet season.

For direr areas the ETc for tuber crops (including shallow root crop) exceeds inflow water for almost four to five months therefore cropping strategies can be applied to compensate this water demand to maximise crop production. On the other hand in wet areas the inflow water (rainfall) exceed ETc of tuber crop all the time around. Wet areas with soils of poor drainage are more vulnerable to water log. Furthermore well drained soils in wet areas are also prone to nutrient leaching. 

These need suitable cropping strategies and crop type to maintain crop production. PNG’s climate varies considerably from year to year due to the El Niño-Southern Oscillation. This regional climate change has two extreme phases, the El Niño and La Niña. El Niño reduces the inflow water for crop production to very low level limit and La Nino receives very high rainfall. These two extreme events normally resulted in excessive and deficit water in the soil storage that causes low crop production and crop failure. 

Under extreme dry scenarios, the inflow water is insufficient for tuber crop in drier areas. ETc exceeds inflow water for 5-8 months that may lead to crop failure for crops depending on rainfall. For wet areas during El Nino rainfall reduced to about 2000mm (National Weather Service data) and thus crop could survive well if proper cropping strategies are applied. 

Under La Nino phase, excessive pouring of rainfall is experienced and low evapotranspiration demand that leads to accumulation of water in the soil storage. Even though rainfall exceeds crop water demand (ETc) but soil volumetric water at saturation is not normally available to crop because the gravitational potential (downward force acting on soil water column) exceeds matric potential (work done to remove water from area of high concentration to low concentration). 

Several global circulation models (GCM) projected the temperature to be increasing in the future and that will also increase evapotranspiration. Hence crop water requirement will increase. 

ETc projected to rise in the future but may be compensated by high precipitation and cloud cover as many other GCM suggested this. Furthermore, the daily crop ETc is constant at different growth stages and rainfall distribution is not normally uniform and that my affect crop production under normal and dry condition. However, the very concern is how to utilise and conserve the limited inflow water or seek alternative water source during moisture deficit scenario and remove and utilise excess water for crop production. With that concern one has to understand the environment within the SPAC.  Within the SPAC during El Nino, inflow water reduces that caused the soil water storage to deplete and continuous evapotranspiration demand. To compensate crop ETc under this scenario the following has to be done currently and during the time of water shortage;  access surface ground water table or existing source, minimise soil compaction and build up soil organic matter content, farming practices to minimise evapotranspiration (mulching, hedgerows etc) and cultivate drought tolerant crops.  On the other hand, under La Nino, excessive inflow water is experienced with low evapotranspiration rate. This accumulates excessive water in the soil storage. 

The cultivation practices used by high rainfall areas like large compost mounds can also be adopted together with the normal practices like rising of beds and drainage; also moisture tolerant crops can be planted.

Generally, not all the inflow water are all available to crop or stored in the soil for later use. It all depends on the soil storage capacity, aggregated and compacted soil, slope (surface flow), deep drainage, texture type and unsustainable farming system. 

For example majority of the soils under specific different agroecological zone in PNG have heavy clay like Yule Island in Central, Murukanam in Madang and Kopafo in Eastern Highlands. These clay soils have micro pores that resulted in poor infiltration and the force of adhesion and cohesion acting on the soil colloid is much greater than the matric potential, that enables the soil to hold more water content but makes it unavailable to plant. The soil to hold better available water capacity is sandy loan, silty loam and loamy soils. These soils are found in the specific study site of Hisiu in Central, Derin in Madang and Tambul in Western Highlands. 

Farming practices in PNG now mainly have to focus on how to improve the soil water storage capacity, minimise evapotranspiration and adopt suitable technology to access ground water or existing water source during dry season or grow drought tolerant crops. On the other hand the farming practices suitable to cater crop production under wet season have to be seriously adopted together with moisture tolerant crops. In addition to that the tall trees and palms have to be maintained around the vicinity of a crop land closer to the coast line for balancing ground water table. This is because sea level rise will raise ground water table and that will convert farm land to swamp land.