The use of potential evapotranspiration (PET) estimates to identify irrigation timing for greenhouse tomatoes (Lycopersicon esculentum Mill.) grown in peat-based substrate was evaluated for a spring and fall crop. PET (using the Penman equation) was calculated from leaf, wet and dry bulb temperatures, and incident and reflected photosynthetic photon flux. Substrate matric potential (SMP) was monitored continuously using electronic tensiometers. Two irrigation starting setpoints (-4.5 and -6.5 kPa SMP) and two nutrient solution electrical conductivity (EC) treatments (1.5 and 3.0 dS·m-1) were factorially combined in a completely randomized design. Irrigation frequency was greater in treatments irrigated at -4.5 than at -6.5 kPa. The integral of calculated PET values was correlated with SMP for both experiments. Accumulated PET values were higher at the start of irrigation in the -6.5-kPa treatments for spring and fall crops. Nutrient solution EC did not influence irrigation frequency. Leaf pressure potential (LPP) was correlated to PET-predicted LPP (r 2 > 0.56) in plants subjected to high EC, low (-6.5 kPa) matric potential setpoint, or both treatments. PET and electronic tensiometer technology can be used jointly to improve irrigation management for tomatoes grown in peat-based substrates by more accurately responding to crop needs for water and nutrients.
Although water conservation programs in the arid southwestern United States have prompted prudent landscaping practices such as planting low water use trees, there is little data on the actual water use of most species. The purpose of this study was to determine the actual water use of two common landscape tree species in Tucson, Ariz., and water use coefficients for two tree species based on the crop coefficient concept. Water use of oak (Quercus virginiana `Heritage') and mesquite (Prosopis alba `Colorado') trees in containers was measured from July to October 1991 using a precision balance. Water-use coefficients for each tree species were calculated as the ratio of measured water use per total leaf area or per projected canopy area to reference evapotranspiration obtained from a modified FAO Penman equation. After accounting for tree growth, water-use coefficients on a total leaf area basis were 0.5 and 1.0 for oak and mesquite, respectively, and on a projected canopy area basis were 1.4 and 1.6 for oaks and mesquites, respectively. These coefficients indicate that mesquites (normally considered xeric trees) use more water than oaks (normally considered mesic trees) under nonlimiting conditions.
In June 1991, a two year field study was initiated to examine if three non-turf groundcovers with reputations for using low amounts of water actually use less water than Kentucky bluegrass (KBG). Irrigation treatments were based on percentages of ET (100%, 75%, 50%, 25%, 0%) and calculated by the modified Penman equation. Results from the 1991 season indicate that at the 100% and 75% treatments Potentilla tabernaemontani and Cerastium tomentosum were significantly better than the other species in terms of establishment and vigor but quality declined significantly at rates below 75%. At the 50% rate both KBG and Sedum acre maintained good quality although growth was slow. At the 25% rate, quality of KBG significantly declined while Sedum acre maintained good quality. Quality of Sedum acre declined only slightly at the 0% treatment and would be a good alternative to KBG if water conservation was a high priority in the landscape.
Mango (Mangifera indica L.) has been grown since the beginning of the century in Baja California Sur, one of the most arid states of México. Since water is a very scarce resource in this area, the estimation of water consumption by popular crops becomes a relevant aspect of hydrological research. Actual (ETa) and potential (ETp) evapotranspiration of mango c.v. Kent were estimated in the Experimental Station of CIB, located 17 Km west La Paz city. Trees under study were three years old; irrigation frequency was 14 days and depth of applied water was 0.15 m, a common amount in the region. Estimates for ETp were carried out through two indirect methods (Blaney-Criddle and Penman equations), and ETa by a direct method (a diffusive porometer). Data were fitted according to the total leaf area (TLA). Estimates from the indirect methods were 31 and 25% respectively greater than those given by the porometer. Main results (ETa = 0.5 cm day-1, or 0.07 m H20/14 days) suggest that a 40 to 50% reduction in the applied water depth is feasible in the region.
Microenvironmental conditions in a shadehouse covered with shade fabric designed to exclude 70% of incoming light were monitored and compared to those in an adjacent field to quantify differences related to plant water use. Radiant flux density and photosynthetic, photon flux inside the shadehouse varied seasonally between about 18% to 28% of outside values. During the day, leaf and air temperatures around the crop canopy were generally lower and relative humidities higher inside the shadehouse than outside. Leaf-to-air vapor pressure gradients inside the shadehouse averaged about half those outside. Wind run inside was <10% of wind run outside. Differences between reference ET (ETo) values, calculated using Penman's equation), inside and outside the shadehouse were greatest during summer months. Outside evaporatory pan (Epan) water losses ranged from 205 mm in July to 95 mm in Nov. For the same months, Epan losses inside were about 80% lower. Monthly ETactual, as determined for Rumohra adiantiformis growing in lysimeters in the shadehouse, ranged from around 40% to 80% of inside Eo.
An automatic irrigation system was designed for use on green-house tomatoes growing in peat-based substrates. This system uses electronic tensiometers to monitor continuously substrate matric potential (SMP) in peat-bags. The system also uses the Penman equation to evaluate potential evapotranspiration (PET) through the acquisition of many greenhouse environmental parameters. Through a series of linear equations, estimates of PET are used in a computer-controller system to vary the electrical conductivity (EC) of irrigated nutrient solutions, as well as SMP setpoints at which irrigations are started. Such modifications to current irrigation management systems may improve fruit quality and reduce the risk of water stress during periods of high PET by irrigating more frequently with less-concentrated nutrient solutions. Conversely, during periods of low PET, irrigation is less frequent with more-concentrated nutrient solutions. Although no differences were found in fruit number or overall yield using variable nutrient solution EC, plant fresh weight was higher in those treatments. It is concluded that an integrated tensiometer-PET system may give increased precision to irrigation management and the control of crop growth in the greenhouse.
Crop evapotranspiration (ETc) was measured as evaporative heat flux from drip-irrigated coffee (Coffea arabica L. cv. Yellow Catuai) fields at different stages of canopy development using the Bowen ratio-energy balance technique. Irrigation requirements were determined by comparing the ETc values obtained against reference values (ET0) derived from a modified Penman equation, and expressed as the ETc/ET0 ratio, or crop coefficient (Kc). In 1991, the average Kc was 0.75 to 0.79 for fields containing 2- to 4-year-old plantings. This ratio was 0.58 for a field containing a 1-year-old planting. Crop coefficient was 30% lower in 1992 due to higher ET0 values and lower stomatal conductance. Measurements made between July and August and again between September and November 1991 suggested that Kc may vary seasonally. Crop transpiration (T), determined with the stem heat balance technique, comprised from 40% to 95% of ETc as the leaf area index increased from 1.4 to 6.7. Behavior of Kc and T during a 25-day soil drying-reirrigation cycle indicated that the crop was able to maintain relatively high levels of gas-exchange activity during periods of severe water deficit.
Accurate irrigation scheduling for sweet corn can reduce irrigation costs and ensure meeting of yield goals. Three scheduling methods, evaluated in a 2-year study, included: a) irrigation when 46% and 57% of available water was depleted in 1984 and 1985, respectively, as measured by a neutron meter; b) irrigation when 50% of available water was depleted as estimated by the Food and Agriculture Organization modified Penman equation; and c) irrigation at three growth stages. Irrigation water applied for the neutron meter, modified Penman, and growth stage method was 367, 279, and 269 mm, respectively, in 1984 while in 1985 these methods resulted in application of 500, 368, and 366 mm of irrigation water. Yields of total unhusked ears in 1984 for the growth stage and modified Penman methods were significantly lower than the yields of the neutron meter method but were not significantly different from one another. In 1985, there were no significant differences in total unhusked or husked processable ear yields among the three scheduling methods. Quality factors, which included ear length, kernel moisture content, and ear weight did not vary significantly with irrigation scheduling methods. Since total unhusked, husked processable yields, and quality differences were minor, irrigation scheduling by any of these methods would appear to be satisfactory.
season to replenish 100% of the weekly estimated evapotranspiration as determined using a weather station (Campbell Scientific, Logan, UT) and a modified Penman equation. Mowing began when irrigation was reduced to once weekly (≈3 weeks after
precipitation during the growing season (1 Mar. to 31 Aug.) of 222 mm. Daily alfalfa-based reference evapotranspiration (ET r ) was calculated using the Kimberly-Penman equation ( Dockter and Palmer, 2008 ; Jensen et al., 1990 ) with weather data taken from an