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Factors Affecting Runoff Nutrient Losses

Important Terms

Table 8.1.1 Key Terms and Definitions

Term Definition

Infiltration Is the process by which water on the ground surface enters the soil. Fissures Physical cracks in soil caused by the loss of water or frost action. Erosive Energy

The ability of flowing water to detach and transport soil particles. The erosive energy of running water depends on its volume and velocity. Surface Runoff

A term used to describe the flow of water, from rain, snowmelt, or other sources, over the land surface. It is part of the water cycle. Water Holding Capacity

Describes the total amount of measurable water that can be retained in a soil profile, and held against gravitational pull.

Runoff is that portion of total precipitation (rain and snow) that does not infiltrate into the soil but instead flows over the soil surface. Snowmelt is responsible for more than 80% of runoff that occurs in Alberta. Runoff can transport nutrients from the field reducing the amount of nutrients available to support crop production. Nutrient-enriched runoff also contributes to accelerated eutrophication of surface water bodies, which can decrease water quality (Chapter 2.3).

Soil nutrients are lost through runoff in two main ways:

Nutrients in soil solution can be lost as dissolved •

forms in runoff. Sediments carried in runoff can transport nutrients •

associated with soil mineral particles or organic matter complexes.

The application of nutrients, in excess of crop requirements, or at times when the crop is not using nutrients leads to accumulations of nutrients that can be lost to runoff. To minimize nutrient losses in runoff, apply manure and fertilizer at rates and times that coincide with periods of greatest crop uptake.

Aside from altering the rate and timing of nutrient application, it is difficult to design strategies that effectively target dissolved nutrient losses. Many of the

practices used to control water erosion can also reduce sediment-bound nutrient losses.

This chapter will look at factors that influence the occurrence and erosive potential of runoff. The two chapters that follow will focus on specific strategies and control measures that target these factors to reduce the risk of nutrient loss in runoff.

Timing, Rate and Form of Precipitation

Precipitation from rainfall and runoff from snowmelt are the driving factors for soil erosion. An understanding of how the timing, intensity and form of precipitation and runoff affect runoff potential will help in the design of strategies to control runoff and water erosion.

Timing and Rate

Timing and rate of precipitation are critical factors affecting runoff. High intensity storms will cause more runoff than low intensity storms. For example, considerable runoff may occur on a site that receives 50 mm of rain in one or two short, severe storms versus if the same volume of rain was to fall on the same site over the course of a week in several intermittent showers.