Estimating the Probability of Break-Even Yields for Continuous Wheat on the Canadian Prairies* R. de Jong, A. Bootsma and J. Dumanski Centre for Land and Biological Resources Research Research Branch, Agriculture and Agri-Food Canada Central Experimental Farm, Bldg #74 Ottawa, Ontario, Canada KlA 0C6 Introduction The purpose of this study was to use long term (30-year) weather records with crop-weather models to estimate yield probabilities and determine how often break-even yields for continuous spring wheat are exceeded under different soil/climatic regimes on the prairies. The broader goal of this type of work is to determine production risks under different management scenarios, such as different crop rotations (we only look at continuous wheat here), different levels of fertility, summerfallowing, minimum tillage, etc. Background information In the prairie region of Canada, crop production is mainly based of dryland farming. Therefore, yields are heavily dependent on the available, but highly variable, moisture supply. The supply of moisture depends on the moisture stored in the soil at seeding time and the amount of rainfall during the growing season. The variability in supply affects the crop water use or actual evapotranspiration, which has been closely linked to crop yield. The prairie region has been divided into several main soil groups, namely the Brown, Dark Brown, Black and Dark Gray Chernozems and some Gray Luvisols (see Fig. 1). These soils form a somewhat concentric pattern around southwestern Saskatchewan and southeastern Alberta, where the driest soil zone (the Brown zone) is located. Water supply generally increases as one moves outward from this area. Soils attributes such as texture, available water-holding capacity and drainage vary within each zone. * Paper presented at the Canadian Meteorological and Oceanographic Society 28th Congress, May 30 to June 3, 1994, University of Ottawa, Ottawa, Canada. A series of yield equations relating the yield of spring wheat to water use or ET have been developed for each of the soil zones by Henry (1990) of the University of Saskatchewan (see Table 1). These equations were based on observations made in the region over a number of years. The equations indicate water-use efficiencies that range from about 9 to 13 kg/ha/mm, with the lowest values found in the driest zones. Figure 2 shows graphically the direct relationship between yield and water use for the 4 soil zones as expressed by these equations. Threshold levels of water use needed before any yield is produced vary from 60 mm in the Brown zone, to 32 mm in the Gray zone. From this it is apparent that if we can estimate the variability in water use, then it is possible to get an estimate of yield variability as well. Model Selection We chose the Versatile Soil Moisture Budget (VSMB) model (Baier et al., 1979) to estimate water use or ET. This budget model calculates a daily water balance, whereby water is gained by precipitation (i.e. rain enters the soil until full capacity is reached) and is lost by runoff, deep drainage and ET. The model has the advantage that it runs all year round and therefore takes into consideration moisture from snowfall which is added to the soil during spring melt. It also has advantages over more physically-based models in that it operates on a daily time step requiring less computer time and has fewer input requirements. Data Bases used as Inputs The model was run on 30 years of daily weather data for about 250 Agroecological Resource Areas (ARAÕs),also known as Land Resource Areas (LRA's) for the prairie region (see Figs. 3a, 3b and 3c). These land areas were distinguished on the basis of agroclimate, surface landform, soil texture and soil development. Each area is considered to be relatively similar in crop production potential, land use and management. The dominant soil textural class in each ARA was used to estimate the available soil water-holding capacity (AWC) as follows: sand - 100 mm; loam - 150 mm; clay loam - 200 mm; clay - 250 mm. Daily weather data were derived from weather station data for the 1956-1985 period using weighting factors determined by the Thiessen polygon weighting procedure and by expert opinion. The daily weather data consisted of maximum and minimum air temperature, precipitation and estimated potential evapotranspiration. Analysis of Results By running the analysis for 30 years, it was possible to determine the cumulative probability distribution for growing season ET. Figure 4 shows the probability that ET values are not exceeded for Saskatchewan ARA #52 in the Brown soil zone, with an assumed AWC of 200 mm. For example, there is about an 80% probability that an ET value of 250 mm is not exceeded (or conversely, a 20% probability that it is exceeded). Table 2 shows the break-even yields in each soil zone which a farmer must obtain to recover the cost of production. Production costs were obtained from provincial Departments of Agriculture. Break-even yields assume 1993 wheat prices of $129/t. Considerable variation in break-even yield can be expected as a result of fluctuating prices from year to year. However, we assumed a fixed price. Based on these assumptions, the break-even yields ranged from 1984 kg/ha in the Brown soil zone, to 2674 kg/ha in the Black and Gray soil zones. Using Henry's yield equations, we were able to estimate the water use or ET needed to achieve these break- even yields as shown in the last column of Table 2. The probability that this water use level is exceeded could then be determined from the 30-yr analysis computed by the VSMB. For example, a water use level of 267 mm must be exceeded to reach break-even yields in the Brown soil zone. Figure 4 indicates that in ARA #52 in Saskatchewan, there is about an 18% probability that this value will be exceeded for a soil with 200 mm AWC. Conversely, the probability of the break- even yield not being exceeded is about 82%. To determine if the probabilities computed by this procedure were realistic, we did a comparison with a cumulative probability function of actual grain yields measured over a 25-yr period (1967-1991) near Swift Current (Zentner et al., 1993), which is near ARA #52 in the Brown soil zone. The probability of exceeding a break-even yield of 1984 kg/ha was about 19% (Fig. 5), compared to 18% for ARA #52. Therefore, the results appear to be reasonable, although further validation of the yield probabilities would be beneficial. As a result of these analyses, we were able to construct a map of the prairie region showing the probabilities of break- even yields for continuous wheat rotation (Fig. 6). The map shows that the probability of break-even yields in the Brown soil zone is less than 20%. This indicates why summerfallowing and flexible cropping are dominant practices in this region. The probabilities increase to 21-60% in the Dark Brown soil zone, 61-90% in the Black and Gray zones. In Table 3, the effects of changes in break-even yields and soil AWC on the probabilities are shown for each of the 4 major soil zones. Break-even yields can change from year to year as the costs of production and wheat prices change. If break-even yields increase by 10%, the probability of excellence drops to less than 10% in the Brown soil zone. Probabilities in the Dark Brown zone would be similar to present values for the Brown zone if break-even yields increase by 20%. Alternately, a 20% decrease in break-even yields would result in probabilities in the Brown zone that are currently for the Dark Brown zone. Conclusions This study has shown that it is possible to estimate the probability of achieving break-even yields for continuous wheat on the Canadian prairies using daily weather data input into crop/weather models. There are several areas towards which future work could be directed. These include: (i) validation of probability estimates of break-even yields for the prairies; (ii) development and use of improved models for estimating yields; (iii) examination of the impact of different management scenarios on break-even yields, such as different crop rotations (this study only examined continuous wheat), minimum tillage and levels of fertility. References Baier, W., Dyer, J.A. and Sharp, W.R., 1979. The versatile soil moisture budget. Tech. Bull. 87, Agrometeorology Section, Land Resource Res. Inst., Agriculture Canada, Ottawa, Ont., 52 pp. Henry, J.L., 1990. Development of crop water production functions for wheat, barley and canola to revise nitrogen fertilizer recommendations in Saskatchewan. Univ. of Saskatchewan, Saskatoon, Sask., Mimeo Report, 16 pp. Zentner, R.P., Dyck, F.B., Handford, K.R., Campbell, C.A. and Selles, F., 1993. Economics of flex-cropping in southwestern Saskatchewan. Can. J. Plant Sci. 73: 749-767.