Em4885 irrigation management practices to protect ground water

3. Soil water conditions - most chemical reactions require some moisture to proceed.
Thus, the amount of soil water can affect the availability of contaminants and the
amount of deep percolation and surface runoff. Deep percolation is the transport
mechanism for leaching chemicals to ground water. Surface runoff can carry
chemicals in solution or soil particles with attached chemicals. Rainfall is hard
to predict and some deep percolation or surface runoff cannot be avoided.
Sound irrigation practices are extremely important for reducing contamination.
4. Soil structure - affects water movement (permeability) through the soil profile.
Potential leaching will increase or decrease depending on the structure.
Soil erosiveness will increase or decrease the possibility of adsorbed chemicals
moving with soil particles that are detached by wind or water.
5. Soil temperature - chemical reactions proceed faster in warm soils. This is
important in mineralizing crop residues to increase the amount of nitrogen
available in the soil. Further, pesticides break down more quickly in warm soils.
9
CHAPTER 3
Chemical application methods affect the pollution process. Desirable management practices are
presented in Chapter 4 with the Overall Management Objectives and Implementation Practices.
Basically chemicals should be applied only when needed and in the minimum amount needed.
Applications should be timed to the irrigation schedule and the irrigation application efficiency
should be as great as feasible.
Irrigation and Rainfall as Detachment and Transport Mechanisms
Water is extremely important in the detachment and transport processes. High flows in furrows
or excessive application rates under a sprinkle irrigation system can cause soil erosion and subsequent
sedimentation. High flows also can transport chemicals attached to the soil particles. Excess deep
percolation can leach nutrients and other chemicals to ground water. Thus, when applying water
carefully consider its fate.
The fate of applied water can be better understood if the hydrologic cycle is understood first.
The hydrologic cycle, illustrated in Figure 3-2, describes the movement of water through its different
forms and places.
Important processes in the hydrologic cycle are:
1. Evaporation - transformation of liquid water into water vapor from free
water surfaces.
2. Precipitation - rain or snow.
3. Runoff - water moving overland or in a river or stream.
4. Infiltration - movement of water into the soil.