In this module, you’ll start your exploration of water with a deeper look at the water cycle on a global scale. You’ll also learn about how water moves through ecosystems and take a look at some important terms and processes that impact how water is managed in agroecostystems.
First off, watch the video below about the global cycling of water. This animation shows one molecule of water completing the hydrologic cycle. Heat from the sun causes the molecule to evaporate from the ocean’s surface. Once it evaporates, it is transported high in the atmosphere and condenses to form clouds. Clouds can move great distances and eventually the water molecule will fall as rain or snow. Ultimately, the water molecule arrives back where it started, at the ocean.
Film 1: The Water Cycle
Here’s a diagram of the water cycle described in the video above.
Here’s a diagram of the water cycle in a rainforest, and nutrient cycle in a rainforest, showing opposite directions of flow:
Movement of Water through Agroecosystems
In natural ecosystems, water enters the system as rainfall or snowmelt at the surface of the soil, or enters aquifers and other groundwater stores to be available to deeper rooted plants. Water continually flows through the bodies of plants, entering through the roots and leaving through the stomata via transpiration. Plants depend on having a certain amount of water available to their roots in the soil. Without adequate moisture, they wilt and die. In natural ecosystems, vegetation is adapted to the soil moisture regime set by the climate and the soil type. However, in agroecosystems, often plants are introduced that have waters needs that exceed the ability of the natural ecosystem to meet those needs, or the ecosystem has too much soil moisture, which can be equally detrimental to the plants. Maintaining sufficient moisture in the soil is a crucial part of agroecosystem management.
Soil moisture management is more than just supplying adequate water by irrigation or rainfall. Soil moisture is part of the whole agroecosystem, and affected by soil structure and soil ecology. Water availability and retention is related to nutrient availability, and as such, is affected by a myriad of factors. In addition, water itself plays many roles: it carries soluble nutrients, affects soil aeration and temperature, and impacts soil biota. A farmer must be aware of how water acts in the soil, how water levels in the soil are affected by weather conditions and cropping practices, how inputs of water affect soil moisture, and what the water needs of crops are.
Water availability in the soil is rarely static: supply fluctuates during a season, and even during a day. Optimum water availability for a particular crop is difficult to determine, and is affected by soil type and structure, as well as other changing factors and conditions. However, we do know the range of moisture conditions that promote high yields for most crops. The challenge for farmers is how to manage water in the soil in ways that keep conditions within this range. Sustainable management of soil moisture depends greatly on understanding the fate and cycling of applied water, with a goal of maximizing efficiency of water use by the system. Below, you’ll look at some important phenomena’s and terms related to the water cycle and management.
Movement through the Water Cycle: Overview of Important Terms
Transpiration: The process known as transpiration is responsible for the movement of water through the plant from roots to small pores on the leaves called stomata (openings in the leaves that facilitates exchange of gases with outside environment). If water is not added to replace the loss of water due to transpiration, plants either go dormant or die and are lost from the ecosystem. In natural ecosystems, some water is returned to the soil during decomposition, but if the plant is harvested from an agroecosystem, some of the water leaves the system when a plant is harvested and sold elsewhere, and the water does not return back into the ecosystem.
Once water enters the soil, it can be lost to the atmosphere through evaporation. Evaporation occurs at the soil surface, but can affect soil moisture deep into the soil profile. Water molecules are strongly attracted to each other, so when evaporation creates a water deficit at the soil surface, molecules are drawn to the surface via capillary action. This process will continue until the saturated zone reaches too deep or the upper soil layer becomes so dry the capillary is broken.
The rate of evaporation from the soil surface depends on the moisture content and temperature of the soil surface. Wind greatly accelerates the evaporation process, especially at higher temperatures. Mulch or soil surface cover slows the heat gain of the soil surface, creating a barrier to prevent or slow the rate of evaporation.
Percolation is the movement of excess water from upper soil surface to deeper in the soil profile. Percolation is determined by soil structure, texture and porosity. Sandy-textured soils have larger pore spaces, and less soil-particle surface area to hold water than more finely-textured soils, and will therefore allow for faster and more percolation. A soil high in fine, clay particle content may allow percolation at first, but depending on the type of clay, attraction between water molecules and clay particles can cause the clay to swell, closing pore space and impeding water percolation. Root channels and animal burrows, especially those of earthworms, are important pathways for percolation. Soil texture and structure, which are heavily affected by tillage, are of greatest importance for percolation.
|Infiltration is the movement of water into the soil. Infiltration is affected by soil type, slope, vegetative cover, and characteristics of the precipitation itself. It is not a given- water can be lost to surface runoff or evaporation if it cannot easily penetrate the soil surface. Water infiltrates more quickly in soils with greater porosity, such as sandy soils or those with a high Soil Organic Matter content. Infiltration and percolation are increased by reduced tillage, increasing SOM content, and improved conditions for soil biota.|
From: Gliessman, Stephen R. Agroecology: The Ecology of Sustainable Food Systems. 2nd ed. Boca Raton: CRC Press, 2007.