Richard L. Snyder

Water balances and evapotranspiration in water- and dry-seeded rice systems

Rice is a crop that is usually grown under flooded conditions and can require large amounts of water. The objective of this 3-year study was to quantify water use in water- (WS) and dry-seeded (DS) systems. In WS systems, the field is continuously flooded, while in DS systems the field is flush irrigated for the first month and then flooded. Research was conducted on commercial rice fields where the residual of the energy balance method using a sonic anemometer and the eddy covariance method were used to determine crop evapotranspiration (ETc) and crop coefficient (Kc) values. In addition, inlet irrigation water and tailwater drainage were determined. Across years, there was no difference in ETc (averaged 862xa0mm), seasonal Kc (averaged 1.07), irrigation water delivery (averaged 1839xa0mm) and calculated percolation and seepage losses (averaged 269xa0mm) between systems. An analysis of the first month of the season, when the water management between these two practices was different, indicated that Kc and water use were lower in DS systems relative to WS systems when there was only one irrigation flush during this period, while two or three irrigation flushes resulted in similar values between the two systems.

Estimation of actual evapotranspiration in winegrape vineyards located on hillside terrain using surface renewal analysis

Sensible and latent heat flux densities were estimated in a level vineyard, a northeast aspect vineyard and a southwest aspect vineyard in the Napa Valley of California using the eddy covariance and surface renewal methods. Surface renewal is theoretically not limited to level or extensively homogeneous terrain because it examines a more localized process of scalar exchange as compared with eddy covariance. Surface renewal estimates must be calibrated against eddy covariance data to account for unequal heating of the air parcels under a fixed measurement height. We calibrated surface renewal data against eddy covariance data in a level vineyard, and the calibration factor (α) was applied to the surface renewal measurements on the hillside vineyards. Latent heat flux density was estimated from the residual of the energy balance. In the level vineyard, the average daily actual evapotranspiration (ETa) for the period of June through September was 2.4xa0mm per day. In the northeast aspect vineyard, the average daily ETa was 2.2xa0mm per day, while in the southwest aspect vineyard it was 2.7xa0mm per day. The net radiation values for the level vineyard, the northeast aspect vineyard, and the southwest aspect vineyard were compared against the Ecosystem Water Program with good agreement.

SIMETAW# - a Model for Agricultural Water Demand Planning

A successful water management scheme for irrigated crops requires an integrated approach, which accounts for water, soil, and crop management. SIMETAW# is a user friendly soil water balance model that assesses crop water use, irrigation requirements, and generates hypothetical irrigation schedules for a wide range of crops experiencing full or deficit irrigation. SIMETAW# calculates reference evapotranspiration (ETo), and it computes potential crop evapotranspiration (ETc), and the evapotranspiration of applied water (ETaw), which is the amount of irrigation water needed to match losses from the effective soil root zone due to ETc that are not replaced by precipitation and other sources. Using input information on crop and soil characteristics and the distribution uniformity of infiltrated irrigation applications in full or deficit conditions, the model estimates the mean depth of infiltrated water (IW) into each quarter of the field. The impact of deficit irrigation on the actual crop evapotranspiration (ETa) is computed separately for each of the four quarters of the cropped field. SIMETAW# simulation adjusts ETo estimates for projected future CO2 concentration, and hence the model can assess climate change impacts on future irrigation demand allowing the user to propose adaptation strategies that potentially lead to a more sustainable water use. This paper discusses the SIMETAW# model and evaluates its performance on estimating ETc, ETa, and ETaw for three case studies.

Weather Station Siting

Temperature is the driving factor in most phenological models, and proper measurement is critical for both development and use of the models. In addition to selecting accurate sensors, they should be mounted at an appropriate height and properly shielded from short-wave radiation (double shielding is best). Choosing small sensors that respond rapidly, protecting electronic leads, and ventilation (in areas with little wind) can improve accuracy of the temperature measurements. Data should be collected at a height that is typical of other weather stations in the area where the model will be used. Generally, agricultural weather stations collect temperature data at 1.5 to 2.0 m height and weather services tend to measure at 10.0 m height. For phenological models of natural vegetation, it is best to site the weather station in a similar environment without irrigation. However, when the models are used for irrigated crops, the stations should be sited over an irrigated grass surface to avoid temperature fluctuations due to intermittent rainfall at the measurement site. Strong temperature gradients can occur near large water bodies (e.g., the ocean or large lakes) and in hilly or mountainous regions where sunlight is blocked during part of the day. In such regions, more weather stations are needed to better characterize microclimate differences. However, even when the temperature data are accurately determined, inaccuracies in model predictions can occur because it is plant temperature rather than air temperature that truly drives the phenological development. Although little or no literature on the topic exists, perhaps using vapor pressure deficits to estimate plant from air temperature could improve models and make them more universally applicable.

Weather Station Siting: Effects on Phenological Models

Accurate temperature data is important for both the development and use of phenological models, and this chapter discusses topics related to temperature measurement for use in phenological models. The chapter presents a short history of temperature measurement, the theory of temperature sensors, radiation shielding, and guidelines on weather sensor placement. Physiological time (degree day) calculation and its application are described. The impact of underlying surface, fetch, and surrounding environment on temperature and phenological time are discussed and some guidelines on measurement are presented.

Wet-bulb, dew point, and air temperature trends in Spain

This study analyses trends of mean (Tm), maximum (Tx), minimum (Tn), dew point (Td), and wet-bulb temperatures (Tw) on an annual, seasonal, and monthly time scale over Spain during the period 1981–2010. The main purpose was to determine how temperature and humidity changes are impacting on Tw, which is probably a better measure of climate change than temperature alone. In this study, 43 weather stations were used to detect data trends using the nonparametric Mann-Kendall test and the Sen method to estimate the slope of trends. Significant linear trends observed for Tm, Tx, and Tn versus year were 56, 58, and 47xa0% of the weather stations, respectively, with temperature ranges between 0.2 and 0.4xa0°C per decade. The months with bigger trends were April, May, June, and July with the highest trend for Tx. The spatial behaviour of Td and Tw was variable, with various locations showing trends from −0.6 to +0.3xa0°C per decade for Td and from −0.4 to +0.5xa0°C per decade for Tw. Both Td and Tw showed negative trends for July, August, September, November, and December. Comparing the trends versus time of each variable versus each of the other variables exhibited poor relationships, which means you cannot predict the trend of one variable from the trend of another variable. The trend of Tx was not related to the trend of Tn. The trends of Tx, Tm, and Tn versus time were unrelated to the trends versus time of either Td or Tw. The trend of Tw showed a high coefficient of determination with the trend of Td with an annual value of R2xa0=xa00.86. Therefore, the Tw trend is more related to changes in humidity than temperature.

Using energy balance data for assessing evapotranspiration and crop coefficients in a Mediterranean vineyard

Improving water use efficiency is a key element of water management in irrigated viticulture, especially in arid or semi-arid areas. In this study, the micrometeorological technique “Eddy Covariance” was used to directly quantify the crop evapotranspiration (ET) and to analyze the complex relationships between evapotranspiration, energy fluxes, and meteorological conditions. Both observed Direct measurements (DIR) of latent heat flux (LE) and observed from the residual of the energy balance (REB) equation were used for crop evapotranspiration calculations. Observed crop coefficients (Kcms) were then determined using the standardized reference evapotranspiration (ETo) equation for short canopies. In addition, linear approximations from observations were used to model the seasonal trend lines for crop coefficients and Kcs values were parameterized by first identifying the beginning and end of each growth stage. The modeled Kcs values were used to predict daily ET from ETo measurements and compared with values from literature. The daily observed DIR ET values (ETdo) were lower than REB ET (ETro) during periods with precipitation, but they were similar during dry periods, which implies that energy balance closure is better when the surface is drier. Comparisons between modeled ET and crop ET estimated using Kc values from best agreement was observed between the modeled REB and FAO 56 and the local Kc values provided by the Regional Agency ARPAS showed good agreement with observed ET (from DIR and REB data) than the FAO 56 ones. The study confirmed that the availability of locally driven Kc could be relevant to quantify the crop water requirement and represents the starting point for a sustainable management of water resources.

Phenology and Evapotranspiration

Many phenological models use ambient air temperature to estimate phenological stages during current and projected future climate conditions. However, the difference between ambient air temperature and plant-canopy temperature biases such estimates. Therefore, consideration of how energy balance factors affect evapotranspiration leads to more realistic plant-canopy temperatures, and better prediction of plant phenological development. Evapotranspiration (ET) has a big impact on the relationship between plant-canopy and air temperature, so awareness of ET facilitates understanding of temperature based phenological models, their limitations, and possible changes in response to climate change. This chapter presents information on the estimation of reference ET (ET o ), applying crop coefficient (K c ) values to determine well-watered crop ET (ET c ), and assessing water stress effects on crop ET. It also discusses how to account for water stress effects to determine actual crop ET (ET a ) and it presents some of the problems associated with estimating the ET of natural ecosystems. The difference between plant-canopy and air temperature is shown to depend on energy balance factors that affect ET. The temperature relationships vary spatially and with time, and therefore using air temperature based degree day models can lead to errors when predicting plant phenological development, which depends on plant-canopy temperature. It is shown that the ET effect on temperature will likely decrease the accuracy of plant phenology models as climate changes.

Technological challenges to irrigation scheduling – Crop Coefficients, ETo forecasts, and in-situ ET measurements

ET-based irrigation scheduling has grown considerably in recent decades as the technology to collect data and disseminate information has improved. A huge advance occurred with the publication of the ASCE-EWRI standardized reference ET (ETref) equations, which provided an equation to account for weather effects on ET by quantifying evaporative demand. Well-watered crop evapotranspiration (ETc) is estimated as the product of ETref and an appropriate crop coefficient (Kc) value that accounts for the difference in ET between the reference and crop surfaces. The estimation of ETc is an important goal for irrigation researchers, but advances are difficult to make because of climate and irrigation management effects on Kc values. For efficient irrigation management, there is also a need to account for water and salinity stress effects on the actual crop evapotranspiration (ETa) when computing irrigation schedules. Many plant physiologists are investigating the use of plant-based measurements for regulated deficit irrigation of orchard and vine crops. ET information, however, is still necessary to properly interpret the plant-based measurements and to modify the irrigation management. In urban irrigation, water is applied rapidly and runoff is often the limiting factor determining irrigation timing and amount. Drip irrigated and shallow rooted crops are also frequently irrigated, so near-real time and forecast ETc estimates are needed to optimize irrigation management. In this paper, information on Kc research in California, ETo forecasts, and in-situ ET measurements is presented.

Assessment of Myrtus communis L. water status and water requirements

Myrtle ( Myrtus communis L.) is a naturally occurring, highly drought-tolerant, evergreen shrub or small tree that is widely distributed within the Mediterranean basin. It is classified as an aromatic species because of the essential oil compounds of the leaves and fruits. Myrtle is commercially used for liquor production and perfume, and the harvest comes mainly from wild plants. Currently, there is insufficient wild-plant production to meet the demand for myrtle leaves and berries which has led to increasing interest to cultivate the plants to increase production. To assess the water requirements and water use efficiency, research was conducted on the ecophysiological responses of commercially grown myrtle plants to different soil-moisture conditions using stem water potential measurements. Data were collected in an eight-year-old myrtle orchard located near Alghero (Italy) during two consecutive summers. Plants showed higher values of net photosynthesis and water use efficiency under moderate stress than in well watered conditions. Severe stress symptoms appeared only with low soil moisture content. These results showed that optimal myrtle production will likely occur using regulated deficit irrigation. Crop evapotranspiration was determined using the surface renewal method, and the crop coefficient (K c ), relative to short-canopy standardized reference evapotranspiration (ET o ) ranged between 0.7 and 1.2.