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Surface Soil Moisture
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SOIL MOISTURE AND CROP PERFORMANCE

The water requirements of a plant depend on their growth stage. A young plant has a shallow root and absorbs much less water than a mature flowering plant with a well extended rooting system. In the growing cycle of an annual crop, these water requirements are well documented and a function of its the seeding date and the temperature. In the case of summer crops it is well possible to forecast yield in comparing water use requirements and the quantity of water available in the soil. For many crops the yield corresponds directly of the relationship between required and available water during a known critical period, usually at the end of the seed development stage. This is documented below.

CROP PERFORMANCE
In DRYMON the performance of crops and natural vegetation is monitored through a so-called Crop Performance Index. The water requirements of a plant depend on their growth stage. A young plant absorbs much less water than a mature flowering plant. In the growing cycle of an annual crop, these water requirements are well documented and a function of its the seeding date and the temperature. In the case of summer crops it is well possible to forecast a yield by comparing its water use requirements and the quantity of water available in the soil. For many crops the yield depends on the difference between required and available water during a known critical period, usually during the seed development stage.

In crop modelling the growth of a plant is simulated, expressing the accumulation of biomass as a function of responses to temperature, sunlight, nutrients and water requirements. Both crop water and nutrients are provided through the soil as an intermediary.
As an example in tropical areas neither light nor (low) temperature are factors that limit plant growth. Nutrients and water however are. In this stage of DRYMON deployment nutrient availability has not been considered, but this may well be addressed in future. Crop water requirements are defined as the depth of water (e.g. in millimetres) needed to meet the water loss through evapotranspiration (which is the combined loss of water from soil and plant to the air from a surface). This factor and a number of others that also are required to model the growth of a plant are crop specific (Doorenbos and Kassam, 1979).

The rainwater that drops on the surface of the soil only in part becomes available to the growth of a plant. It evaporates, runs off or runs on from somewhere else or infiltrates. The infiltrated water, that does not seep into the groundwater and that is not too strongly attached to the soil particles, is the soil water available for the plant growth. This available soil water is of course also a function of the rooting depth of the plant, which increases rapidly during the first weeks after germination. This available soil water can also be expressed as a depth of water for a certain soil type and location. The methods used to simulate the behaviour of water in a soil and its external drainage are called 'a soil water balance'.

 
In this diagram for a millet crop on a loamy sandy soil two water balances are presented over a growing season. The lines represent a diagram for the situation in Segou in the year 1992. The green water balance line represents the water requirements for the plant as simulated by a so-called 'evapotranspiration-driven model', while the orange line represents a 'SCAT-driven model'. The blue and the red line represent boundary conditions respectively above which and below which the plant cannot grow because there is too much or too little water. In this case the SCAT model indicates the start of the growing season much better than the evapotranspiration-driven model, especially the dry spell around week 26.
 
In this animation the surplus and water deficit is illustrated for in 1999 for sorghum in West Africa. The surplus/deficit diagram for week 36 illustrates how variable a drought is: Western and Southern Mauritania has dry cropping conditions in the critical period while in Mali very good yields may be expected.