Versatile Soil Moisture Budget Research Articles

Observations and modelling of soil water content changes around trees

This paper presents the results of a 5-year study of water content variations in a clay soil due to tree roots and evapotranspiration. The site of the study program is Maisonneuve Park which is located in the eastern area of Montreal, in the heart of a zone known for settlement problems experienced by lightweight buildings supported on shallow foundations. The clay deposit around several trees (both isolated and in a row) has been monitored by means of several arrays of 3–5 m long thin aluminium tubes, spaced from 0.1H to about 3H from the trees, where H represents the height of the trees. The volumetric water contents around the tubes have been measured by means of a neutron probe. The study shows that while for some trees, the effects of the roots are confined to a volume of soil located at a distance less than the height of the tree, in other cases, the zone affected by the trees is of much greater extent, reaching as far as 1.5H. In addition, for some of the trees studied, their effect extends to depths exceeding 4 m. Water content changes have also been estimated using the Versatile Soil Moisture Budget model, which was developed at Agriculture Canada. Comparisons between soil moisture readings in the field with estimates from the model show the feasibility of estimating soil moisture changes from standard meteorological data. Key words: water content changes, clay deposit, tree roots, evapotranspiration, observations, modelling.

ESTIMATED SNOWMELT INFILTRATION AND RUNOFF FOR THE PRAIRIE PROVINCES

A revised version of the Versatile Soil Moisture Budget model, which included an algorithm for infiltration into frozen soils, was utilized to simulate snowmelt infiltration and runoff for the agricultural region of the Prairie Provinces of Canada. Thirty years of daily data from 78 climatological stations within the Prairie region were used in the model to produce maps of the average annual winter precipitation, snow water equivalent, and runoff for fallow and stubble lands.The average annual water equivalent of snowcovers within the region ranged between 25 mm and 60 mm on fallow land and between 35 mm and 85 mm on stubble land. Snowmelt infiltration did not vary widely throughout the region ranging from 17 to 23 mm for fallow and from 23 to 33 mm for stubble. Average annual snowmelt runoff ranged between 10 mm and 40 mm for fallow and between 15 and 55 mm for stubble. Average annual runoff for the three years with highest runoff was approximately double that of the 30-year average.

Estimation of snowmelt runoff in the Peace River region using a soil moisture budget

Rainfall and snowmelt runoff on soil frozen below the surface are recognized as important factors contributing to soil loss in Canada. The risk of rain on frozen soil has been quantified, and the amount of snowmelt on frozen soil has been estimated. This study extends such research to derive a climate-based model to estimate winter and spring runoff. This could result in a more accurate erosion prediction for areas where snowmelt is a major source for runoff. Selected components of the Water Erosion Prediction Project (WEPP) model and the versatile soil moisture budget (VB) were tested on observed data for two study sites in the Peace River region. The version of the WEPP model available to us estimated snow depth, soil frost depth and frequency of freeze–thaw cycles. However, the results did not adequately match observed data. The VB was modified in this study to improve the estimate of potential winter and spring runoff, and it was shown that incorporating observations of snow depth improved the estimate of the time and amount of snowmelt runoff. The modified runoff model was validated with data collected in the Peace River area of northern Alberta and British Columbia and with published data from the Prairies. Key words: Snowmelt, runoff, soil moisture budget

Relation between the normalized difference vegetation index and ecological variables

The objectives of this study were to analyze the relationship between the AVHRR data [transformed into a normalized difference vegetation index (NDVI)] and ecological variables and, secondly, to evaluate the possibility of estimating actual evapotranspiration using AVHRR and other readily available data. The study was undertaken across a range of conditions in Canada. Meteorological and composited NDVI data were compiled for 15-day periods over the 1986 growing season and were supplemented by digital ecological and soil data. The effect of soil and meteorological conditions on vegetation growth was modeled using the Versatile Soil Moisture Budget model, which accounts for additions and losses of water from various layers in the root zone. The various data sets were integrated using a geographic information system. We found that individual vegetation/soil combinations exhibited different NDVI trajectories, most closely related to the trends in potential evapotranspiration. There was a highly significant association between coincident NDVI and actual evapotranspiration values ( r = 0.77) or between NDVI and potential evapotranspiration for the previous 15-day period ( r = 0.86). High correlation was found between cumulative NDVI and cumulative actual evapotranspiration over the growing season ( r = 0.96). Actual evapotranspiration for individual 15-day periods could be estimated from NDVI and potential evapotranspiration values with a residual error of about 10–15%. The results show that the NDVI trajectories provide important information on the seasonal development of vegetation at northern latitudes. Results in estimating actual evapotranspiration were also encouraging but further work is needed to determine the accuracy limits for this geographic area. The representativeness of the 1986 results for the region is subject of continuing study with 6 years of NDVI data.

ESTIMATED LONG-TERM SOIL MOISTURE VARIABILITY ON THE CANADIAN PRAIRIES

Soil water contents under wheat production systems on the Canadian prairies were estimated using the Versatile Soil Moisture Budget, which was modified to include an estimated seeding date and a biometeorological time scale. Large year-to-year variability required the use of long term (60 yr) data to describe seasonal and probable occurrences of soil water contents for 27 stations. Regression and correlation analyses were used to simplify presentation of the data and to display their spatial distribution. On average, summerfallowing was most effective in conserving water in areas with intermediate spring soil water contents and on soils with a higher available water-holding capacity (AWC). In abnormally dry years (i.e., 10% probability) summerfallowing conserved more water in the wetter areas while increasing the AWCs had the greatest effect on conserved water in the intermediate areas. Soil water contents at heading were greater under wheat-fallow rotation compared to continuous wheat and increased with increasing AWCs. Water contents of less than 50% of AWC at heading occurred with a probability of 80% or higher in most areas, indicating some stress is experienced in most years. Summerfallowing lowered the probability only slightly on soils with a low AWC, but by 10–20% on the higher AWC soils. The probability of extreme water stress at heading (i.e. water contents ≤ 10% of AWC) ranged from 0% in the wettest areas to 40–60% in the driest climatic area. Key words: Soil water content, versatile soil moisture budget, summer fallowing, zonation, wheat (spring), seeding date

COMPARISON OF TWO SOIL-WATER MODELS UNDER SEMI-ARID GROWING CONDITIONS

The Versatile Soil Moisture Budget (VSMB) and the Soil-Plant-Air-Water (SPAW) model were compared and tested against soil-water content data from a long-term experiment in which wheat was grown on fallow land in the semi-arid zone of Saskatchewan. Both models were driven by daily air temperatures and precipitation data. At the expense of requiring more detailed soil and crop information, the SPAW model simulated the water balance and its components in greater detail than the VSMB. Predictions of soil-water contents throughout the growing season with either model corresponded very well with the measured data. The overall mean absolute difference in total soil-water content to a depth of 120 cm was 1.5 cm for the VSMB and 1.2 cm for the SPAW model. Predictions of water distribution in the profile were also satisfactory. A choice between the two models, to be used under semi-arid growing conditions, will depend on the availability of input data and the required level of output. Key words: Soil-water movement, modelling, evapotranspiration

Soil water modelling using daily and mean‐daily data derived from historical monthly values*

Abstract Two soil water models, the Versatile Soil Moisture Budget and the Aridity Index Model were used to investigate differences in modelling results as a consequence of using as input mean‐daily data, derived from historical monthly values, instead of actual daily data. Four different available water‐holding capacities, six different initial soil water contents and 30‐year precipitation and potential evapotranspiration data from 16 climate stations across Canada were used as input to the models. Using mean‐daily data as opposed to daily data resulted in model predictions that underestimated deep drainage and aridity indices and overestimated actual evapotranspiration. The differences in model output decreased when the available water‐holding capacity increased and the initial soil water content decreased. The use of mean‐daily data in the soil water models investigated is not recommended, until improved techniques that retain the characteristics of the highly variable weather conditions can be ascerta...

Potential Future Developments in Soil Moisture Budget Models for Humid Regions in Canada

The concept of the daily soil water budget was examined. A number of budgets which fit this class of models were reviewed, but particular emphasis was given to the Versatile Soil Moisture Budget (Baier and Robertson, 1966). The strengths and weaknesses of currently available models of this type were discussed. The most significant weakness is the failure to consider excess soil water in many of these models. It is recommended that the essential components in soil water budget models, which make them applicable to a broad range of soils and climates, should account for (a) soil water retention characteristics, (b) water table depth, (c) infiltration and drainage rates, and (d) snow cover. Submodels for these components from a number of currently available budget type models are discussed, and some modifications are proposed. Some field research requirements are also identified. The development of an integrated soil water budget, with submodels for all of the above components is recommended, but each new su...

ESTIMATED SOIL WATER RESERVES APPLICABLE TO A WHEAT-FALLOW ROTATION FOR GENERALIZED SOIL AREAS MAPPED IN SOUTHERN SASKATCHEWAN

Long-term mean soil water reserves for a spring wheat-fallow rotation in the southern half of Saskatchewan were calculated using the Versatile Soil Moisture Budget. Four different available water-holding capacity classes and climatic data from 53 stations were used as input to the model. Soil water reserve data for the following times, seeding on 1 May in the crop year, at heading on 30 June, and on 1 May in the fallow year, were mapped. These were then combined with an available water-holding capacity map to portray in a single map the combined droughtiness due to climatic and soil attributes. Estimated soil water reserves compared well with measured data from one location in the Brown soil zone. The temporal and spatial changes in water reserves are discussed and related to summerfallowing. The maps provide information for use in making potential grain yield estimates. Key words: Soil water, wheat-fallow rotation, generalized soil areas, Saskatchewan, Versatile soil moisture budget

INTERMITTENT INFILTRATION AND EVAPORATION FROM SOIL COLUMNS

Varying total amounts of water (160 and 320 mm) were infiltrated into 60-cm columns of air-dry saline sodic clay soil. The intervals between irrigation applications were varied from 5 to 20 days. The soil columns were subjected to a potential evaporation rate of 4.8 mm∙day−1 in a growth room. The cumulative evaporation followed a square root of time response, similar to that found by others for non-saline soils of coarser texture. An analytical solution of the Richards’ equation gave satisfactory (± 10%) prediction of cumulative evaporation at the end of the experiment as long as water was added in amounts of 40 mm or more per irrigation. The numerical solution to the Richards’ equation gave satisfactory estimates of evaporation for the latter stages of the experiment, but in the earlier stages it underestimated evaporation because of the too deep distribution of water in the soil given by this model. The neglect of hysteresis was invoked to explain the discrepancy between observed and predicted soil water content profiles. The "versatile soil moisture budget" empirical model also gave satisfactory prediction of evaporation but the successful prediction of water content profiles depended on "field capacity" values measured in situ. Key words: Soil moisture, modelling, water budgets, Richards’ equation

The soil moisture regime under native grassland

Detailed measurements of the soil moisture regime under untreated native grassland are reported over a 4-year period and compared with similar data for fertilized grassland, burned grassland, patches of buckbrush and wheat on fallow. Water use of the untreated native grassland ranged from 21.5 to 35.5 cm; fertilization with nitrogen increased water use by about 1 cm during the year of application but had little or no residual effect. The major effects of burning were a reduction in soil moisture recharge during the winter period and lower water use in the year following the burn. Average water use of the buckbrush patches was about 4 cm higher than that of the surrounding native grassland. Water use of wheat grown on fallow was similar to that of native grassland in one out of three years, but in the other two years was several centimeters lower. Over the 4-year period, average water use (29.4 cm) of the native grassland accounted for about 90% of the annual precipitation (32.6 cm). The remaining 10% is probably lost by evaporation during late fall, winter and early spring, and perhaps some runoff during snowmelt. The water use data for the untreated native grassland were related to aboveground dry matter production ( r = 0.99). The production of untreated native grassland is compared with long-term data for wheat on stubble. The Versatile Soil Moisture Budget was used successfully to simulate the moisture regime of the untreated grassland as it is affected by soil, climatic, and vegetation properties. The relationship between soil temperature and root activity is discussed in some detail.

The performance of soil moisture estimates as compared with the direct use of climatological data for estimating crop yields

Wheat yields from 39 plantings across Canada during five seasons were more closely related to estimated soil moisture than observations of rainfall, maximum and minimum temperatures. Daily soil moisture estimates in each of six zones were obtained from a previously described versatile soil moisture budget (VB), which requires as input only standard climatic data, tabulated astronomical values and some knowledge of moisture characteristics of the soils. The better performance of the soil moisture model as compared with the direct use of climatological data for estimating grain yields was demonstrated by higher correlation coefficients, lower coefficients of variation and lower standard errors of estimate, when estimated yields from the statistical models were correlated with actual yields. Multiple correlation analysis indicated that soil moisture as estimated by the described technique was the best estimator of crop yields, followed in order by minimum temperature and maximum temperature. Rainfall per se was found to be unsuitable as basis for yield estimation.