Water field capacity

Location Soil series pHb Organic matter % Field capacity % water... 

Other models successfully employ a simple water routing system. Each layer of soil is assumed to hold all water entering the layer up to the field capacity. When the water content of a soil layer exceeds the field capacity, water drains downward to the next layer at the rate specified by the hydraulic conductivity of the saturated soil in the layer. 

Different kinds of plants vary somewhat in their ability to extract water from a dry soil, but these differences are not great. Plant tolerance of drought conditions is a much more important factor. Another important factor in the utilization of soil water is the matter of where the water is located. In soils having moisture contents of less than field capacity water movement is much too slow to meet the requirements of plants that are transpiring rapidly. The rate of capillary movement of water through soils, either horizontally or vertically, is much less rapid than many have supposed. Since the water cannot move readily toward the roots, it is necessary that the roots ramify throughout the soil if the soil water is to be used fully. Water in a reservoir moves readily, but water in soil that is held by capillarity is largely immobile. 

Fig. 6 Three-phase distribution and available water at the field capacity water content of SIG-mIxed sandy soil.

Because many pesticides are appHed to the soil surface, the transport of pesticide during water infiltration is important. Water infiltration is characterized by high initial infiltration rates which decrease rapidly to a nearly constant rate. Dry soils have greater rates of infiltration than wet soils during the initial appHcation of water. Thus, perfluridone movement after appHcation of 3.8 cm of water was considerably greater in soil at a water content of <1% of field capacity than at 50% of field capacity (62). Fluometuron moved deeper into the soil in response to greater rainfall intensity or after rainfall onto a dry rather than a moist soil (63). 

For all the essential nutrient ions, the diffusion coefficient, Du is essentially the same with a value of around 10 cm s whereas the water flux at the root surface is typically of the order 10 cm s for soils at around field capacity. The tortuosity factor typically scales with the volumetric moisture content over quite a wide range of moisture content, i.e., / 0. As the soil becomes drier, the water flux will decline much faster than the tortuosity factor due to the typi-... 

For these reasons, it is desirable to perform a series of simple calculations to determine if the field capacity for a given depth of soil is ever exceeded, rather than simply overlaying water inputs over plots of residue data. The following series of calculations addresses the primary issue of whether sufficient water was applied to the test system at appropriate intervals to create leaching opportunities ... 

More sophisticated methods that actually measure volumetric water content can also be used, such as time domain reflectometry (TDR). In Figure 14, an example of TDR results is presented. Both the calculated and measured (i.e., TDR) volumetric water contents provide a similar picture of the profile water status by depth with time. Proper soil characterization data, such as those shown in Table 6, are necessary for these calculations and improve understanding of the test system. The determination of water-holding capacity (WHC) at 0.03 MPa field capacity (FC) and 1.5 MPa... 

Soil strength Water-holding capacity Field capacity/wilting point Hydraulic conductivity Fertility... 

Table III illustrates the impact of adsorption on the leaching of organic chemicals in the soil. A water input of 305 cm was used, which is equivalent to a full year of precipitation in the eastern United States. In a soil with a field capacity of 30%, the water would penetrate 1017 cm. Mirex with a very large Kqc is practically immobile after a full year of precipitation, it is still on the surface. It is likely that any compound adsorbed this strongly would be carried off the land surface by soil erosion instead of being leached into the soil. In contrast, DBCP, which is very weakly adsorbed, penetrates the soil profile almost as far as the water does.

Saturation percentage-amount of water to saturate 100 g of soil (approximately twice field capacity). 

Soil solution to soil ratios also strongly affect distribution of some trace elements such as Zn speciation in arid and semi-arid soils. Fotovat et al. (1997) reported that the proportion of free hydrated Zn2+ to total Zn ranged from 20-65% at field capacity soil water content and decreased with increases in solution to soil ratios, while the proportion of Zn complexed with organic ligands increased dramatically in soils. However, solution to soil ratios do not strongly affect the distribution of Cu speciation in soil solution since Cu primarily occurs as organic complexes in these soil solutions. 

Figure 6.6. Long-term changes of Uts of metals in a loessial soil from Israel. The soil received metal nitrates under field the capacity moisture regime (after Han and Banin, 1999. Reprinted from Water Air Soil Pollut, 114, Han F.X and Banin A., Long-term transformations and redistribution of potentially toxic heavy metals in arid-zone soils. II Incubation under field capacity conditions, p 245, Copyright (1999), with permission from Springer Science and Business Media)...

Han F.X., Banin A. Long-term transformations and redistribution of potentially toxic heavy metals in arid-zone soils. II Incubation under field capacity conditions Water Air Soil Pollut 1999 114 221-250. 

SOL AWC (mm/mm) Available water capacity of the soil layer. The available water in the soil is calculated by subtracting the water content at the permanent wilting point from that at field capacity SOL AWC = FC - WP. 

If it is necessary or desirable, water can be extracted from unsaturated soils in the laboratory. This requires either pressure or suction to move water from the soil. A common laboratory method for removing water from unsaturated soils is the pressure plate (see Figure 7.13). Plates for this type of extractor can be used for extraction of water at field capacity 33 kPa and at permanent wilting point - --1500 kPa.3... 

The water fraction of each element of soil can not exceed a certain maximum value, known as the field capacity. If the water content is less than the field capacity, then no water is able to leave the element. If the water content is equal to or greater than the field capacity, then water is able to leave the element at the same rate as water enters. The water is also taken up by the roots of plants. Here it is assumed that the uptake rate of water is proportional to the length of root. If the water content falls below a critical value, the plants can no longer take up water, and they wilt and eventually die. 

In practise some water will always be transported through the soil even if the local water content is less than the field capacity owing to preferential flow through cracks and large pores in the soil structure. This can be modelled simply by defining a bypass parameter to account for the fraction of water that can pass through each element. Modify the model to include bypass and see how this influences the solute profiles (see Corwin et al. (1991) for further details). 

Different soil types have differing field capacities and minimum water contents. 

Soil type Field capacity Minimum water content... 

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