Strategies to Improve Water Use Efficiency: Part 2

Part 1 of this series covered crop water balance and strategies to improve water use efficiency, part 2 covers transpiration and soil moisture content.

In Part 1, we introduced the crop water balance equation:

Precipitation (P) + Irrigation (I) = Runoff  (R) + Deep Drainage (D) + Evaporation (E)+ Transpiration (T) ± Soil Water Content (∆S).

The water balance equation considers the crop and soil as a box with inputs and outputs, the inputs being P+I and the outputs R+D+ET and ∆S soil water in the root zone a storage component. The components of the water equation are expressed in units of depth (such as inches or cm of water). In part 1 we discussed R, D and E, and today we will consider T and ∆S.

T  -  Transpiration.

This is the water that is taken up by the plants and contributes to crop production. Transpiration by the crop should be maximized, while transpiration by undesired plants should be eliminated. Weed control helps to avoid weeds from using precious soil moisture. Optimum nutrient availability helps crops use moisture for growth. Some crops require less moisture than others. C-4 crops like corn, sorghum, sudangrass and pearl millet are examples of crops that use water very efficiently and are therefore well adapted to warm and dry conditions. However, we also know that corn is very sensitive to drought at tasseling. Corn is also less adapted to drought than sorghum or sudangrass which have the ability to close their stomata (openings in the leaves that allow for exchange of water and carbon dioxide) for a while without adversely affecting production. Alfalfa is a crop notorious for its high water use – however, it often does not suffer from drought as much as one would expect because of its very deep rooting system. Winter crops such as wheat or barley grow during times when soil water is typically plentiful. Combining summer and winter crops in crop rotations is a good way to 'armor' your farm against summer drought by spreading risk. Farmers can also hedge against drought by planting hybrids or varieties bred for greater drought-resistance. Early planting of summer crops is another important drought evasion strategy for our state. The root system of an early planted corn crop, for example, is much bigger and deeper than that of late-planted corn when dry periods hit so the early planted corn will show less drought stress although the larger crop uses more water.

∆S  - Soil moisture.

Soil moisture content fluctuates during the season. In Pennsylvania, soil moisture availability is especially impacted by soil type. A soil may have large quantities of coarse fragments limiting total soil moisture storage (such as shaly soils) or be shallow which doesn't allow for a deep root system (some of our limestone soils are shallow, while other soil types may have compact layers such as fragipans). Thus a farmer with a deep soil can sustain 3 weeks of dry weather, while a neighbor with shale soil can only last for a week before crops start to wilt. It may not be possible to change these inherent soil characteristics, but it is important to adjust your management to your soil type. For example, it may not make sense to grow corn on these droughty soils, but sudangrass could be a better choice. Winter wheat is also better adapted to droughty soils because it grows when soil moisture is typically more plentiful. Further, it is important to monitor spring moisture conditions on droughty soils to determine cover crop termination date – on these soils it is best to terminate the cover crop earlier if dry weather is in the forecast in the late spring to avoid soil moisture depletion by the cover crop. The importance of mulch cover is, however, very important in summer, and that would justify using planting green into large cover crops if spring moisture is plentiful, because the large amount of mulch helps conserve soil moisture in the summer.  Early planting of corn is also important on these soils to allow for deep rooting and hopefully have corn tassel before summer drought hits. Planting in the optimum window helps deep rooting while late planting results in a shallow root system in the summer. Soil water storage can also be improved by improving soil health. Increasing the organic matter content of your soil improves soil water holding capacity. USDA-NRCS estimates that 1% organic matter increases soil moisture holding capacity greater than 20,000 gallons per acre. Making your soil more porous by favoring soil biological activity and having continuous living roots in the soil increases soil water penetration and improves root exploration of the following crop. Finally, surface and subsoil compaction should be avoided because it reduces the root system and limits water uptake.

In part 3, we evaluate how managing the water balance will become more important in the future of farming because of changing temperature and precipitation patterns.