Agroclimatic Atlas of Alberta: Soil Moisture Conditions in Alberta

This document is part of the Agroclimatic Atlas of Alberta.

Soil Moisture in Agriculture

Soil moisture is ultimately the source of water for crops because virtually all water used by crops is taken from the soil by the root system. Whether the water source is precipitation or irrigation, for practical purposes the water must first enter the soil before the crop can use it.

Since precipitation during the growing season in Alberta is not sufficient to meet crop demand, agriculture relies on water stored in the soil from fall and winter precipitation to supplement what the growing season provides. The soil moisture reserve also provides a steady supply of moisture that allows the crop to grow during moderately dry periods.

The level of soil moisture reserve in the spring is an indicator of risk to crop production. When soil moisture levels are drier than normal, timely above-normal precipitation is required during the growing season to make up the deficit. The chance of this occurring in a growing season is low, which means the risk of below-average crop production increases. When moisture reserves are high, crops can produce normal yields despite below-average precipitation and periodic dry spells, and will generally produce above-average yields if the growing season precipitation amounts are average or better. Therefore, the risk of below-average crop performance is reduced. Droughts occur in Alberta when precipitation is not sufficient to return soil moisture to normal levels, or to meet normal crop demands during the growing season, or both.

Not all moisture in the soil is available for crop growth. Crops can access about half of the total moisture in the soil. The rest is too tightly held within the soil for plants to use. For agricultural purposes, soil moisture refers to the plant-available water. Plant-available water is the fraction of the total soil water available for plant use. It is usually expressed as an amount per depth of soil (e.g. millimetres per metre) for convenient relation to precipitation amounts.

Moisture Storage Efficiency

Soils vary in their capacity to store soil moisture, with sandy soils having about half the capacity of clay soils (Table 4). Therefore, if the top 120 cm (the root zone) of a sandy soil and a clay soil were both completely filled with soil moisture, the clay soil would be able to supply over 200 mm of plant-available water, whereas the sandy soil would provide only about 100 mm. This means that risk of poor crop performance due to water shortage is higher on sandy soils than on clay soils.

Table 4. Plant-available water holding capacity for soil textural classes

Mapping Soil Moisture

Alberta Agriculture, Food and Rural Development samples and maps soil moisture as part of an effort to monitor the risk of agricultural drought. Annual fall soil moisture maps were produced from 1982 to 1994, and then were reinitiated in 2001. Preparation of annual spring soil moisture maps began in 1988 and is ongoing.

The maps describe the conditions for medium textured soils on annually cropped land (stubble fields), with irrigated land excluded. They are based on a set of 250 to 300 representative fields. Soil samples are taken to a depth of 100 cm in each field during October for the fall maps and during April for the spring maps.

Soil moisture varies from one part of a field to another, and locations a few metres apart can have considerable differences in moisture levels. Differences are caused by uneven snow cover, variable patterns of snowmelt and runoff, and variation in precipitation, topography, cropping patterns, land management, and soil properties such as texture. Even small surface depressions, not noticeable when fields are dry, can hold considerable water during snowmelt. It is difficult to characterize the moisture content of a field using a single moisture content; therefore, the maps are based on ranges of moisture content.

Annual soil moisture levels are classified as Very Low, Low, Medium, or High, which correspond to ranges of 0 to 25, 25 to 75, 75 to 125 and over 125 mm of plant-available water, respectively. To assess long-term trends, average soil moisture levels for spring and fall are determined based on the midpoint of each range.

Spring soil moisture is an indicator of risk for successful cropping in the upcoming growing season. Lower than average soil moisture levels mean that crops are more dependent on the right amounts and timing of growing season precipitation, and the risk of poor crop performance from water stress increases. Above-average soil moisture levels mean that crops can survive short or moderate dry periods during the growing season, and the potential for average or better crop performance increases.

Figure 10 shows the average spring soil moisture levels from 1988 to 2002. The driest springs occur in an area east of Calgary to Medicine Hat and Coronation (165 km east of Red Deer). A large portion of eastern Alberta and part of the central Peace Region have spring moisture levels as dry as those in the Lethbridge - Medicine Hat area.

Annual re-cropping would be successful in most areas of Alberta if 100 mm of soil moisture could be stored by May 1, given normal growing season precipitation. In the drier areas of Alberta, overwinter precipitation is not always sufficient to provide enough soil moisture for successful re-cropping. In areas where irrigation is not available, one management option is to not plant a crop (summerfallow) in alternate years. The non-cropping season is extended to about 20 months from about eight, and this is usually long enough to provide sufficient soil moisture to successfully produce a crop.

Crop residue management practices to conserve soil moisture are key to crop production in low moisture areas. Such practices include reduced or zero tillage and snow trapping. Managing crop rotations to optimize the crop's use of soil moisture and flexcropping (basing the decision to seed or summerfallow on spring soil moisture levels) are other options for managing crops under limited moisture conditions.

Figure 11 shows the per cent of years from 1988 to 2002 in which 100 mm of plant-available water was observed in the soil in the spring. South central Alberta, from Calgary to Medicine Hat and north to Coronation, has the lowest probability of 100 mm of spring soil moisture. Other low areas include pockets in the northeast portion of the agricultural area and in the central Peace Region. Much of southwest and west central Alberta can expect 100 mm of spring soil moisture at least 80 per cent of the time.

Fall soil moisture is an indicator of potential shortages of livestock water supply, because livestock are often watered from dugouts, which are refilled by runoff from snowmelt. Dry conditions prior to winter tend to decrease snowmelt runoff, because the snowmelt will more likely infiltrate into the soil than flow as runoff into a dugout. Fall soil moisture is typically lower than spring soil moisture in Alberta.

Figure 12 shows the average fall soil moisture levels from 1982 to 1994. The driest area extends from east of Calgary to the southeast corner of the province and north to Provost (about 240 km east of Red Deer). The wettest area is west central Alberta, with a small pocket west of Red Deer having the highest moisture levels. Most areas in the northeast portion of Alberta's agricultural area and in the Peace Region have fall soil moisture levels as low as those near Lethbridge.

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