Water Use and Yield Response of Potatoes

Transcription

1 172 Water Use and Yield Response of Potatoes Carl Shaykewich, Department of Soil Science, University of Manitoba, Winnipeg, MB R3T 2N2 Richard Raddatz, Meteorological Service, Environment Canada Guy Ash, Crop and Weather Surveillance, Canadian Wheat Board Randall Renwick, Department of Soil Science, University of Manitoba Dale Tomasiewicz & Staff, Manitoba Crop Diversification Center, Carberry, MB This paper is intended to provide some basic information on the water use and yield response of potatoes to water supply, i.e. precipitation and/or irrigation. It begins by describing some of the basic principles governing water use by crops, and presents results of a 5-year study of water use and yield response conducted at the Manitoba Crop Diversification Centre at Carberry. These results are used to illustrate some important principles governing the water relationships of potatoes. Basic Principles of Water Use by Crops Water use is composed of evaporation from the soil surface and transpiration from plant leaves. Evaporation from soil is equal to evaporation from a water surface when soil is wet but decreases very rapidly as soon as soil surface dries out. Thus, evaporation is not a major contributor to water use by crops. The bulk of crop water use is by transpiration, i.e. evaporation from plant leaves. Both evaporation and transpiration depend upon weather. Since it takes energy to evaporate water, the most important component of weather is solar energy sunshine. Another important weather element is temperature the higher the temperature the greater is water loss. Other weather elements of importance are relative humidity lowering humidity increases water loss; and wind higher winds cause surfaces to lose water faster. Transpiration depends upon some other additional factors. The first of these is the portion of ground covered by green leaves, since it is only from these surfaces that water can be lost. Water loss from leaf surfaces is decreased by moisture stress, which is primarily dependent upon the soil moisture content in root zone. In this regard, it is important to remember that early in the growing season the crop may be under moisture stress even if there is a lot of moisture at lower depths because roots may not have grown deep enough to reach that water. Studies at Carberry During the summers of a study consisting of growing potatoes under non-limiting soil fertility, sprayed to control blight, and irrigation added to maintain soil at pre-determined moisture levels was conducted at the Manitoba Crop Diversification Centre at Carberry. The main objectives of the study were 1) to track water use by potatoes over the growing season and determine if it could be accurately estimated by a model developed at Environment Canada, and 2) to determine the effect of water supply on yield. Two varieties of potatoes, Shepody and Russet Burbank were used. Measurements of root growth, % of ground covered by leaves, and soil moisture were used to test and fine-tune the water use model. The effect of water supply on yield was assessed by measuring marketable yield. Evaluation of Water Use Model The water use model was evaluated by comparing measured to modelled available soil water in the root zone throughout the growing season. This assessment showed that on average modelled values were about 3 mm higher than measured, and the root mean square error was about 13 mm. It was concluded that this was sufficiently accurate to justify the use of the model for irrigation scheduling.

2 173 Water Demand by Potatoes Having concluded that the model gave a reasonable estimate of soil moisture, and therefore must have estimated water use fairly accurately, the model was used to estimate water demand throughout the growing season. A graphical representation of accumulated water demand and accumulated precipitation is shown in Fig 1. Water demand is low until about the middle of July because area of leaves covering the ground is low 2-3 mm (1/8 inch) per day. After that, combination of higher temperatures and more complete ground cover increases water demand to as much as 8-10 mm (1/3 inch) or more per day. This is sharp contrast to cereal crops, which are maturing at this time year, and their water use drops off sharply. Water demand drops off in September due to decreasing temperatures and solar energy. Carberry 1996 mm water Water Demand Precipitation May 23- May 1-Jun 10- Jun 19- Jun 28- Jun Figure 1. Accumulated precipitation and water demand by potatoes at Carberry during the 1996 growing season. To grow a crop of potatoes in Manitoba, it takes 375- mm (15 16 inches) of water to completely avoid water stress. To get some idea of the degree of water stress likely to be experienced by potatoes, this demand can be compared to growing season precipitation at several Manitoba locations (Table 1). 3-7-Jul 16- Jul 25- Jul Date Sep 17- Sep 26- Sep Table 1. Average and one in four year risk values for precipitation during the growing season of potatoes. Location Growing Season Precipitation (mm) Average One year in four risk Portage la Prairie 265 or less Carberry or less Graysville or less Morden or less Emerson or less Pilot Mound or less The table shows that on average we get about mm (10 inches) of precipitation during growing season. Thus, on average we need 125- mm (5-6 inches) of additional moisture to avoid water stress. If we

3 174 consider the one in four year risk for dry years, we need at least - mm (6-8 inches) additional moisture. On the other hand, if we look at the one in four year risk for wet years, we need mm (1-3 inches) or less additional moisture. Some of that will be supplied by stored soil moisture at planting. Exact amount depends upon fall and winter precipitation and the soil s ability to hold water. Since Maximum rooting depth of potatoes is about 90 cm - almost 3 feet, the available water holding capacity of that depth of soil is the appropriate soil characteristic (Table 2). Table 2. Average amount of Available Water to a Depth of 90 cm in Textural Groups of Manitoba Soils. =========================================== Available Water mm Inches Soil Textural Group in in 3' (USDA Classification) 90 cm Profile Sand Loamy sand Sandy loam Loam Clay Loam Clay On average, there will be about 100 mm of available water in the top 90 cm of the soil suitable for potatoes at planting in Manitoba. Because rooting depth is very shallow in the early part of the growing season, plants can experience water stress even though there may be lots of moisture in soil below the root zone. Thus, in dry springs, it may be necessary to irrigate even though soil moisture status may have been very good to start with. Variation of Water Supply with Season in Carberry Studies One of the objectives of the Carberry studies was to evaluate the effect of water supply on yield. To that end, four treatments were used: A) Soil irrigated when root zone moisture dropped to 75% water holding capacity, B) Soil irrigated when root zone moisture dropped to 55% water holding capacity, C) Soil irrigated when root zone moisture dropped to 35% water holding capacity, and D) rainfall only. In each treatment, soil moisture was measured at weekly intervals. The results of those measurements for 1996 are shown in Figure 2. The numbers at the end of the trace of each soil moisture regime are yield, expressed as cwt/ac. The driest soil water regime, i.e. (D) rainfall only, yielded 301 cwt/ac for Shepody and 313 cwt/ac for Russet Burbank. Yields steadily increased with each increase in soil moisture. At the wettest soil water regime (A) irrigated when soil moisture dropped to 75% - yields for Shepody and Russet Burbank were 378 and 394 cwt/ac, respectively. As seen from Figure 1, the 1996 season precipitation was close to the long-term average for Carberry. Thus, the graph illustrates the kind of benefits that can be derived from irrigation.

4 175 Percent Available Moisture in the Root Zone with Date by Irrigation Treatment and Variety, MCDC Experiment, Carberry, Available Moisture (%) RB A RB B RB C RB D Shep A Shep B Shep C Shep D Jun 6-Jul 13-Jul 20-Jul 27-Jul Sep 14-Sep 21-Sep Date Figure 2. Variation in available soil moisture in the 1996 growing season for Russet Burbank (RB) and Shepody (Shep) for the four irrigation treatments. Results presented in Figure 2 suggested that yields could be estimated if water supply was known. To test this possibility, the graphs shown in Figures 3 and 4 were prepared Shepody y = x x R 2 = Precipitation + Irrigation (mm) Figure 3. Marketable Yield of Shepody as affected by water supply (precipitation + irrigation). 954

5 Russet Burbank y = x x R 2 = Precipitation + Irrigation (mm) Figure 4. Marketable Yield of Russet Burbank as affected by water supply (precipitation + irrigation). 954 An illustration of the interpretation of the equations in the graphs might go as follows: increasing water supply, i.e. precipitation + irrigation, from mm (12 inches) to 325 (13 inches) increase marketable yield of Shepody from 330 to 346 cwt/ac, and yield of Russet Burbank from 310 to 322 cwt/ac. In the graphs, the R 2 values indicate the proportion of the variation in yield that can be related to water supply. Estimation of yield for Shepody was relatively satisfactory almost ¾ of the variation in yield was explained by a relationship to precipitation plus irrigation. Results with Russet Burbank were not as good only a little more than half of the variation in yield was explained by a relationship to precipitation plus irrigation. Shepody likely reached close to full maturity in all years of the study; Russet Burbank probably did not because it requires a longer growing season. A method of characterizing length of growing season for potatoes is P-days a heat unit specific to potatoes. We tried to improve estimation of yield by plotting yield against (precipitation + irrigation) x P- days. The results of those efforts are shown in Figures 5 and 6.

6 Shepody y = -9E-09x x R 2 = Water Use x P-days Figure 5. Relationship of marketable yield to water use and P-days for Shepody. Russet Burbank y = -9E-09x x R 2 = Water Use x P-days Figure 6. Relationship of marketable yield to water use and P-days for Russet Burbank. As expected, adding the effect of P-days did not materially improve yield estimation of Shepody only about 2 % more yield was explained. With Russet Burbank, there was about a 7% improvement in yield estimation by adding effect of P-days. In this case the inclusion of P-days into the estimation was worth doing. Equations estimating yield were used to create tables (Tables 3 and 4) estimating yield from precipitation + irrigation and P-days.

7 178 Table 3. for Shepody predicted from seasonal P-day accumulation and the sum of precipitation + irrigation (mm). ====================================================== Precipitation P-days + Irrigation (mm) Table 4. for Russet Burbank as function of seasonal P-day accumulation and the sum of precipitation + irrigation (mm). ======================================================= Precipitation P-days + Irrigation (mm) Just to put these tables in perspective, most of Manitoba south of the lakes and east of the escarpment has an average annual accumulation of Pdays, while the south east and areas to the west of the escarpment have average of P-days. Summary The main points of this presentation may be summarized as follows: 1. In Manitoba, a crop of potatoes can use up to 375- mm (15-16 inches) of water. 2. On average, we get about mm (10 inches) during the growing season. One year in four we get mm (8 inches) or less. 3. Depending on fall and spring rainfall, we may have mm (2 to 4 inches) of plant available water in the soil at planting. 4. To eliminate water stress, we need to irrigate; the exact amount depends on year. Probably one year in four we need about 100 mm (4 inches). 5. The yield increase due to increase in water supply is probably about 20 cwt/ac per 25 mm (1 inch).