Maize Evapotranspiration Research Articles

Evapotranspiration of irrigated and rainfed maize–soybean cropping systems

We have been making year-round measurements of mass and energy exchange in three cropping systems: (a) irrigated continuous maize, (b) irrigated maize–soybean rotation, and (c) rainfed maize–soybean rotation in eastern Nebraska since 2001. In this paper, we present results on evapotranspiration (ET) of these crops for the first 5 years of our study. Growing season ET in the irrigated and rainfed maize averaged 548 and 482 mm, respectively. In irrigated and rainfed soybean, the average growing season ET was 452 and 431 mm, respectively. On average, the maize ET was higher than the soybean ET by 18% for irrigated crops and by 11% for rainfed crops. The mid-season crop coefficient K c (=ET/ET 0 and ET 0 is the reference ET) for irrigated maize was 1.03 ± 0.07. For rainfed maize, significant dry-down conditions prevailed and mid-season K c was 0.84 ± 0.20. For irrigated soybean, the mid-season K c was 0.98 ± 0.02. The mid-season dry down in rainfed soybean years was not severe and the K c (0.90 ± 0.13) was only slightly lower than the values for the irrigated fields. Non-growing season evaporation ranged from 100 to 172 mm and contributed about 16–28% of the annual ET in irrigated/rainfed maize and 24–26% in irrigated/rainfed soybean. The amount of surface mulch biomass explained 71% of the variability in non-growing season evaporation totals. Water use efficiency (or biomass transpiration efficiency), defined as the ratio of total plant biomass ( Y DM) to growing season transpiration ( T) was 5.20 ± 0.34 and 5.22 ± 0.36 g kg −1, respectively for irrigated and rainfed maize crops. Similarly, the biomass transpiration efficiency for irrigated and rainfed soybean crops was 3.21 ± 0.35 and 2.96 ± 0.30 g kg −1. Thus, the respective biomass transpiration efficiency of these crops was nearly constant regardless of rainfall and irrigation.

Evapotranspiration and crop coefficient of spring maize with plastic mulch using eddy covariance in northwest China

Spring maize under plastic mulch is the staple food crop in northwest China. Studying its evapotranspiration (ET) and crop coefficient ( K c) is important for managing water-saving irrigation in the region. Eddy covariance (EC) was applied to measure spring maize ET in 2007 in northwest China, focusing on the characteristics of the maize ET and K c processes under plastic mulch. An interesting result was that a higher K c in this study relative to the value of FAO 56 was presented in the mid and late season, e.g. average K c was 1.46, 1.39 and 1.22 during the heading, filling and maturity stage, respectively. This result was mainly due to that (1) the plastic mulch had an effect on anti-senescence of maize and great green leaf still existed before the harvest; (2) the FAO 56 PM model may underestimate the reference crop ET in the mid and late season of maize in the region; (3) the planting density was higher in the study, which was about 374,800 plants ha −1. Though K c during the mid and late season was high, a high water use efficiency of 25.2 kg ha −1 mm −1 was still obtained in the study. Our study confirmed that plastic mulch has beneficial effect on improving maize water use efficiency in this severe water shortage region of northwest China.

Comparison of estimates of daily solar radiation from air temperature range for application in crop simulations

Daily solar radiation is an input required by most crop growth, development and yield simulation models. It is, however, not observed at many locations, preventing the application of such models. The objective of this work was to (i) evaluate several existing models estimating solar radiation based on daily minimum and maximum air temperature and/or precipitation for seven sites in the world, and (ii) investigate the impact of the estimated solar radiation on grass reference evapotranspiration (ETo) and total plant dry biomass simulations for maize. Comparisons of the solar radiation models was made based on a single modular indicator, I rad, computed using a fuzzy expert system that aggregated several statistical indices, and distribution of mean daily errors over a year. The model estimations were also evaluated according to their ability to simulate ETo and total dry biomass that matches simulations from the observed solar radiation. According to the I rad indicator, there was no solar radiation model which consistently outperformed all the other models at all the sites tested, but I rad indicated models that relatively underperformed at all the sites. The graphical presentation of the mean fluctuation of errors over a year gave a good assessment of the solar radiation estimation models in investigating the temporal behaviour and magnitude of the residuals. Performance of the models according to I rad and simulations of grass ETo and total dry biomass agreed better for models that relatively underperformed. Ranking of the models according to the root mean square error (RMSE) in solar radiation estimation and the RMSE in the grass ETo simulations agreed very well. Comparison of the ranking of the models using the I rad (or the individual statistical indices thereof) and total biomass simulation was difficult because of the difference in the time scale used in calculation of the statistical indices. The difference in simulations of total dry biomass accumulated over the years, however, qualitatively agreed with the graphs of the mean fluctuation of errors over a year. In general, the I rad indicator demonstrated which solar radiation estimation models should be used for crop simulation modelling.

Water requirements and irrigation scheduling of spring maize using GIS and CropWat model in Beijing-Tianjin-Hebei region

Due to the over use of available water resources, it has become very important to define appropriate strategies for planning and management of irrigated farmland. In this paper, Beijing-Tianjin-Hebei (Jing-Jin-Ji) region was chosen as the case study area for its special political and economic status and its severe water problem. To achieve effective planning, the information about crop water requirements, irrigation withdrawals, soil types and climatic conditions were obtained in the study area. In the meantime, a GIS method was adopted, which extends the capabilities of the crop models to a regional level. The main objectives of the study are: 1) to estimate the spatial distribution of the evapotranspiration of spring maize; 2) to estimate climatic water deficit; 3) to estimate the yield reduction of spring maize under different rainfed and irrigated conditions. Based on the water deficit analysis, recommended supplemental irrigation schedule was developed using CropWat model. Compared to the rainfed control, the two or three times of supplemental water irrigated to spring maize at the right time reduced the loss of yield, under different scenarios.

Effects of harvest and sowing time on the performance of the rotation of winter wheat–summer maize in the North China Plain

Rotation of winter wheat ( Triticum aestivum L.) and summer maize ( Zea mays L.) is the prevailing double-cropping system in the North China Plain. Typically, winter wheat is planted at the beginning of October and harvested during early June. Maize is planted immediately after wheat and harvested around 25th of September. The growing season of maize is limited to about 100–110 days. How to rectify the sowing date of winter wheat and the harvest time of summer maize are two factors to achieve higher grain yield of the two crops. Three-year field experiments were carried out to compare the grain yield, evapotranspiration (ET), water use efficiency (WUE) and economic return under six combinations of the harvest time of summer maize and sowing date of winter wheat from 2002 to 2005. Yield of winter wheat was similar for treatments of sowing before 10th of October. Afterwards, yield of winter wheat was significantly reduced ( P < 0.05) by 0.5% each day delayed in sowing. The kernel weight of maize was significantly increased ( P < 0.05) by about 0.6% each day delayed from harvest before 5th of October. After 10th of October, kernel weight of maize was not significantly increased with the delay in harvest because of the lower temperature. The kernel weight of maize with thermal time was in a quadratic relationship. Total seasonal ET of winter wheat was reduced by 2.5 mm/day delayed in sowing and ET of maize was averagely increased by 2.0 mm/day delayed in harvest. The net income, benefit–cost and net profit per millimetre of water used of harvest maize at the beginning of October and sowing winter wheat around 10th of October were greater compared with other treatments. Then the common practice of harvest maize and sowing winter wheat in the region could be delayed by 5 days correspondingly.

Effect of Irrigation to Winter Wheat on the Soil Moisture, Evapotranspiration, and Water Use Efficiency of Summer Maize in North China

A double-cropping system such as that of winter wheat and summer maize in north China requires irrigation to achieve production goals. The soil during the seeding stage of summer maize is usually dry. For summer maize, soil moisture before sowing (SMBS) is important for seedling growth, yield, and water use. Our objective was to investigate the effects of applying different irrigation schedules to winter wheat on the SMBS, yield, and water use efficiency (WUE) of summer maize. Field experiments involving winter wheat with 0, 1, 2, and 3 irrigation applications (T0, T1, T2, and T3, respectively) were conducted in a dry year (2002), moderate year (2003), and humid year (2005). Irrigation of winter wheat increased the average SMBS of summer maize in the 1.2 m soil profiles to 17.7, 57.3, and 79.6 mm in T1, T2, and T3, respectively. Differences in the soil moisture content among the treatments were significant in the late growing season of summer maize in the dry and moderate years but not in the humid year. In 2005, maize yields among the treatments were not significantly different as compared to those in T0; the yields increased from 22.1% to 56.0% in 2002 and from 1.9% to 11.4% in 2003. Compared to T0, the WUE of summer maize increased by 15.3%, 23.6%, and 24.7% in T1, T2, and T3, respectively, in 2002 and by 0.01%, 3.4%, and 5.4% in 2003; however, it decreased by 2.4%, 5.6%, and 12.4%, respectively, in 2005. Considering the combined WUE of winter wheat and summer maize, there was no significant difference between T2 and T3 (LSD, P < 0.05). These results suggest that winter wheat should be irrigated at the jointing and heading stages to achieve reasonable grain yield and WUE for both the crops.

An evaluation of EPIC soil water and yield components in the gully region of Loess Plateau, China

The Erosion and Productivity Impact Calculator (EPIC) has been used to determine the effect of different cropping systems and management practices on soil productivity in the Loess Plateau of China. However, its crop growth and soil water balance submodels have not been verified in this region. The objective of the present study was to evaluate the ability of EPIC to estimate soil water content (θ in m3/m3), seasonal evapotranspiration (ET in mm/season) and crop yield (Y in t/ha) for winter wheat and maize. A 20-year field experiment was conducted at the Changwu Agro-ecological Experimental Station of the Loess Plateau, and divided into a calibration period and a validation period. Data from calibration (1984–94) were used to optimize the four most sensitive parameters of the EPIC crop yield submodel, whereas data from 1994 to 2004 were used for validation. For both crops, there were no significant differences between measured and estimated long-term means of the three variables (P=0·05) for either the calibration or validation periods. EPIC estimated all three variables with a small relative root mean square error (RRMSE), i.e. the ratio of root mean square error to the mean value. For wheat and maize, the calibration period resulted in respective RRMSE values of 0·112 and 0·100 for θ, 0·121 and 0·116 for ET, and 0·135 and 0·147 for Y. During the validation period, the RRMSE values obtained were 0·090 and 0·085 for θ, 0·129 and 0·135 for ET, and 0·169 and 0·149 for Y, for wheat and maize, respectively. The performance of EPIC in estimating annual values of θ, ET and Y was variable. For validation, EPIC explained 65, 79 and 64% of the measured variations of θ, ET and Y, respectively, for wheat, and 60, 70 and 67% for maize. The EPIC-estimated long-term average values of the three variables were not significantly different from measured values for winter wheat and maize during the calibration and validation periods. It can therefore be used in the gully region of the Loess Plateau to define alternative cropping systems and management practices.

Simulation of groundwater dynamics in the North China Plain by coupled hydrology and agricultural models

AbstractWe simulated the effects of irrigation on groundwater flow dynamics in the North China Plain by coupling the NIES Integrated Catchment‐based Ecohydrology (NICE) model with DSSAT‐wheat and DSSAT‐maize, two agricultural models. This combined model (NICE‐AGR) was applied to the Hai River catchment and the lower reach of the Yellow River (530 km wide by 840 km long) at a resolution of 5 km. It reproduced excellently the soil moisture, evapotranspiration and crop production of summer maize and winter wheat, correctly estimating crop water use. So, the spatial distribution of crop water use was reasonably estimated at daily steps in the simulation area. In particular, NICE‐AGR reproduced groundwater levels better than the use of statistical water use data. This indicates that NICE‐AGR does not need detailed statistical data on water use, making it very powerful for evaluating and estimating the water dynamics of catchments with little statistical data on seasonal water use. Furthermore, the simulation reproduced the spatial distribution of groundwater level in 1987 and 1988 in the Hebei Plain, showing a major reduction of groundwater level due mainly to overpumping for irrigation. Copyright © 2006 John Wiley &amp; Sons, Ltd.

Simulating the effects of extreme dry and wet years on the water use of flooding-irrigated maize in a Mediterranean landplane

The effects of years of extreme rainfall events on maize water-use under traditional flooding irrigation in a Mediterranean landplane were estimated through a simulation assessment; combining a weather generator with an agrohydrological simulation model. Two options: “Fully Irrigation” and “Deficit Irrigation” were considered in the simulations as the extreme water-management situations. Besides, a 2 m depth shallow water table and Free Drainage were considered as the typical extreme situations that can be found at the bottom of the simulated soil layer. Thirty “Dry” (DY) and “Wet” (WY) years were randomly selected from the weather generator output. The model SWAP was used to simulate the relative transpiration (RT), i.e. ratio between actual and maximum maize transpiration, actual maize evapotranspiration (ET C), percolated water and capillary rising during wet and dry years and for each of the irrigation and bottom condition options. According to the modelling results, average mean RT is about 80% and 90% in dry and wet years, respectively. RT and ET C variability are very high under dry conditions although such variability is notably reduced if a suitable irrigation option is considered. Capillary rising can play a very important role during dry years in those places where irrigation is not enough, but water table is relatively shallower. On the other hand, a shallower water table can carry out RT reductions during wet years, due to water excess, although these negative effects are comparatively lower than those produced by rain scarcity. Besides, percolated water during wet years is very high, particularly in well-irrigated farms.

Canopy photosynthesis, evapotranspiration, leaf nitrogen, and transcription profiles of maize in response to CO2 enrichment

AbstractThe effects of CO2 enrichment on the growth and physiology of maize were investigated at the molecular, biochemical, leaf, and canopy levels. Maize plants were grown in sunlit soil–plant–atmosphere research (SPAR) chambers at ambient (370 μmol mol−1) or elevated (750 μmol mol−1) atmospheric carbon dioxide concentration (Ca) under well‐watered and fertilized conditions. Canopy gas exchange rates and leaf temperatures were monitored continuously during the growing season. CO2 enrichment did not enhance the growth or canopy photosynthesis of maize plants. However, canopy evapotranspiration rates decreased by 22% and daytime leaf temperatures were increased about 1°C in response to CO2 enrichment. Leaf carboxylation efficiency and leaf nitrogen concentration also decreased at elevated Ca. Transcription profiling using maize cDNA microarrays revealed that approximately 5% of tested genes responded to CO2 enrichment. Of the altered transcripts, several were known to encode proteins involved in stomatal development or photosynthesis. For the majority of the altered transcripts, however, it was difficult to link their functions with specific physiological factors partly because many of these genes encoded unknown proteins. We conclude that maize did not exhibit enhanced growth or photosynthesis in response to CO2 enrichment but a number of molecular and physiological processes including those involved in stomatal relations were affected by growth in elevated Ca.

Water, heat fluxes and water use efficiency measurement and modeling above a farmland in the North China Plain

Net radiation ( R n ), water vapor flux ( LE ), sensible heat flux ( H s ) and soil heat flux (G) were measured above a summer maize field with the eddy-covariance technique, simulation and analysis of water, heat fluxes and crop water use efficiency were made with the RZ-SHAW model at the same time in this study. The results revealed significant diurnal and seasonal variability of water vapor flux for summer maize. Most part of R n was consumed by the evapotranspiration of the summer maize. The proportion of water vapor flux to net radiation (( LE/R n ) increased with the crop development and peaked around milk-filling stage with a value of 60%, a slightly lower than that obtained by the RZ-SHAW model. Daily evapotranspiration estimated by the model agreed with the results measured with the eddy-covariance technique, indices of agreement (IA) for hourly water vapor fluxes simulated and measured were above 0.75, root mean square errors (RMSE) were no more than 1.0. Diurnal patterns of H s showed the shape of inverted ”U” shifted to the forenoon with a maximum value around 11:30 (Beijing time), while LE exhibited an inverted “V” with a maximum value at around 13:00, about an hour later than H s . Diurnal change of CO 2 showed an asymmetrical “V” curve and its maximal rates occurred at about 11:30. Variations of water use efficiency during the phonological stages of the summer maize showed a rapid increase with the photosynthetic photon flux density (PPFD) after sunrise, a state of equilibrium around 10:00 followed a decrease. Maximum values of water use efficiency were 24.3, and its average value ranged from 7.6 to 10.3 g kg -1 .

Prediction of crop yield, water consumption and water use efficiency with a SVAT-crop growth model using remotely sensed data on the North China Plain

A process-based crop growth model is developed to predict regional crop yield, water consumption and water use efficiency (WUE) using remotely sensed data for a portion of Hebei province (88 779 km 2), most of which is located on the North China Plain (NCP). Model inputs consist of Geographic Information System (GIS) maps of land use, Digital Elevation Model (DEM), soil texture, crop canopy leaf area index (LAI) which is retrieved from the 10-day maximum composite National Oceanic and Atmospheric Administration (NOAA)–Advanced Very High Resolution Radiometer (AVHRR) data, and daily interpolated meteorological variables. The model is run at 92 367 30″ × 30″. resolution grids at an hourly time-step for energy balance and daily time-step for crop growth simulation. Simulated winter wheat and summer maize yields in 1992 and 1993 are compared with both the point samples and the county-level census data. For the 108 counties, the root mean square error (relative deviation) is 1124 kg ha −1 (23%) for winter wheat and 1359 kg ha −1 (33%) for summer maize, respectively. Spatial patterns of simulated crop yield and water use efficiency are strongly influenced by irrigated/rainfed conditions. The modelled grain yield for irrigated winter wheat ranges from 3900 to 7200 kg ha −1, which is significantly larger than when only rainfed, 100–2600 kg ha −1. The modelled grain yield for irrigated maize ranges from 5800 to 8600 kg ha −1, which is significantly larger than when only rainfed, 1400–4800 kg ha −1. This suggests that the potential exists to increase yield in this region, if sufficient irrigation water is supplied. However, given the over allocation of limited surface water, an increase in irrigation is unlikely, and increasing importance will be placed on maximizing regional WUE over the NCP. The water consumption (defined here as modelled evapotranspiration (ET)) for winter wheat ranges from 330 to 500 and 70 to 280 mm for irrigated and rainfed conditions, respectively. The modelled ET for irrigated maize ranges from 350 to 520, and 140 to 350 mm for rainfed conditions. The simulated WUE ranges from 12.25 to 15.75 kg ha −1 mm −1 for irrigated winter wheat and from 0.5 to 8.25 kg ha −1 mm −1 in rainfed areas. The simulated WUE ranges from 11 to 19.25 kg ha −1 mm −1 for irrigated maize and from 5 to 11 kg ha −1 mm −1 in rainfed areas. These, together with the results of simulated crop yields, are comparable with previous studies in the NCP, other parts of China, and internationally, indicating the potential to apply this model to other agricultural regions. This study implicates a new view for regional agricultural and water resources management by assessing regional crop yield, water consumption and WUE consistently based on modelling biophysical processes with the aid of remote sensing.

Modeling soil-root water transport with non-uniform water supply and heterogeneous root distribution

A 2D physically based framework is proposed to analyze the effect of a non-uniform water supply at the soil surface generated by rainfall interception and stemflow on soil-root water transport in the case of heterogeneous distribution of the roots in the soil profile. To model soil-root water transport, the root water potential of two plants placed in two adjacent rows was simulated so as to minimize the difference between the evaporative demand and the amount of water taken up by each plant. A characterization of the throughfall to incident rainfall, soil hydrodynamic properties, soil-root contacts, and maize evapotranspiration, was carried out during a 10-day experiment with a leaf area index of about 4 to 5 m2 m−2. Mean rainfall interception percentages were in the [47.4%–52.6%] range at half the distance between two adjacent rows, whereas an interception percentage higher than 80% was found near the stems along the rows. As a result, the mean estimated stemflow was 1 L per plant per 16.4 mm water supply above the canopy. Good agreement was found between the measured and predicted transpiration values. As the soil started to moisten, the predicted root water potential rapidly increased, in line with the predicted number of active roots that rapidly decreased. Effects due to stemflow during infiltration disappeared progressively when drying was in progress. The proposed approach could be useful for analyzing soil-root water transport and possible pollution when solutes move with water under various realistic conditions where non-uniform water supply is involved.

Determination of evapotranspiration for maize and berseem clover

Maize and berseem are among the most important crops in India and several other countries in the world. Irrigation is provided to these crops to get higher production; hence, determining the water requirements of these crops is important for irrigation planning. Improved water management of these crops requires accurate scheduling of irrigation, which in turn requires accurate measurement of crop evapotranspiration (ETc). Thus, the first objective of this study was to measure daily, weekly and seasonal ETc of maize and berseem directly from weighing type lysimeters. Experiments were conducted in a set of two electronic weighing-type lysimeters of 7.82 m3 to measure the hourly ETc of maize and berseem from June 1996 to April 1998 at Karnal, India. The average daily ETc of maize varied from 4 mm day–1 at development and reproductive stages. The peak daily ETc of maize was 7.7 mm day–1 and this occurred 9 weeks after sowing (WAS) at the silking stage of maize when leaf area index (LAI) was 5.5. The measured seasonal ETc of maize was 354 mm. In the case of berseem, the average daily ETc was 0.9 mm day–1 at the initial stage, achieved a peak value of 6.9 mm day–1 between 25 and 26 WAS during the fifth cut. The measured seasonal ETc of berseem was 480 mm. Precise information on the crop coefficient, which is required for regional-scale irrigation planning, is lacking for semi-arid climates such as those found in north India. Therefore, the second objective of this study was to develop crop coefficients (K c) for maize and berseem from ETc measurements and weather data. The estimated values of K c for maize by the Penman–Monteith method at the four crop growth stages; namely, initial, crop development, mid-season and maturity, were 0.55, 1.00, 1.23 and 0.64, respectively, and the corresponding values for berseem were 0.76, 0.82, 1.11 and 1.24, respectively. In the case of these two crops, actual K c values determined from this study are different from those suggested by the FAO (Allen et al. 1998), indicating the need for generating these values at the local/ regional level.

Crop coefficient and ratio of transpiration to evapotranspiration of winter wheat and maize in a semi-humid region

Crop coefficient ( K c) is an important parameter for irrigation scheduling and water allocation. This study investigated and determined the crop coefficients and the ratio of transpiration to evapotranspiration ( T p /ET c) of winter wheat ( Triticum aestivum L.) and maize ( Zea Mays) based on lysimeter data for 10 years in a semi-arid region of northwest China (34°20′N, 108°24′E). Several relationships, K c and days after sowing (DAS), K c and leaf area index (LAI), and T p/ET c and LAI, were also analyzed. The average seasonal ET c for winter wheat and maize were 443.6 and 424.0 mm, respectively, compared to their reference crop evapotranspiration (ET 0), 483.9 and 407.7 mm, respectively, for their seasons. The seasonal transpiration accounted for 67.0 and 74.0% of their evapotranspiration for winter wheat and maize, respectively. The average, maximum and minimum seasonal K c were 0.92, 1.33 and 0.42, respectively, for winter wheat, and 1.04, 1.43 and 0.45, respectively, for the maize. The relationship between K c and DAS was fitted to a fifth-order polynomial equation with significant correlation coefficients, R 2=0.960 and 0.980, respectively, for wheat and maize. The maximum K c occurred at about 200 and 80 DAS for winter wheat and maize, respectively. K c and LAI showed a saturation relationship, R 2=0.861 and 0.952, respectively, for wheat and maize, and K c exceeded 1.0 when LAI was larger than 2.5 for both crops. The T p/ET c and LAI also showed saturation relationship, R 2=0.968 and 0.981, respectively, for wheat and maize. T p/ET c started from 0 at sowing, and reached to its maximum, near 0.9, at the middle growth stage or when LAI reached to about 3.0 for both crops. T p/ET c maintained at its maximum when LAI was larger than 3.0. These results should help the precise planning and efficient management of irrigation for these crops in this region.

Effects of Hail on Evapotranspiration and Plant Temperature of Maize

AbstractThe effect of natural and artificial hail on the evapotranspiration of maize (Zea mays L.) was studied. In 1998, observations were made after a natural hailstorm, while in 1999 hail was simulated by lacerating the leaves parallel with the leaf veins. Plants of one treatment were grown in a lysimeter. In addition to evapotranspiration, measurements were also made on plant temperature. Among physiological processes, the modifications occurring in evapotranspiration could be divided into two distinct phases. On the days immediately following the damage the raised plant temperature caused by the wounds indicated a reduction in evapotranspiration leading to less efficient plant cooling. Later, in both the natural and artificial hail treatments, changes in evapotranspiration depended on the weather. The changes in evapotranspiration and plant temperature after natural and artificial hail injury were similar in nature. The yield loss was less than 10 %, in agreement with data in the literature on the effect of simulated hail injury. However, it can be expected that further study of simulated hail injuries will lead to a better understanding of the changes in plant water balance caused by hail.

Determination of daily evaporation and evapotranspiration of winter wheat and maize by large-scale weighing lysimeter and micro-lysimeter

Daily evapotranspiration of irrigated winter wheat ( Triticum aestivum L.) and maize ( Zea mays L.) were determined for five seasons between 1995 and 2000 using a large-scale weighing lysimeter, and soil evaporation for each crop was measured for one season using two micro-lysimeters at Luancheng Station in the North China Plain. The results showed that total water consumption averaged 453 and 423 mm for winter wheat and maize grown without water deficit. The water consumption of winter wheat during its growth period greatly exceeds the precipitation, which ranges from 50 mm in dry years to 150 mm in wet years. Consequently, supplemental irrigation is very important to winter wheat production in the region. The average crop coefficient during the whole growth period was 0.93 for winter wheat and 1.1 for maize. Evaporation from the soil surface took up 29.7 and 30.3% of the total evapotranspiration for winter wheat and maize, respectively, equaling an annual loss of more than 250 mm water. Thus, reducing soil evaporation could be one of the most important water-saving measures in this serious water deficit region. Leaf area index (LAI) and moisture in the surface soil greatly affect the ratio of soil evaporation to total evapotranspiration. The relationship between this ratio and surface soil moisture and leaf area index was established, and can help to improve field water utilization efficiency.

Anthropogenic land-use change in the North American tall grass-short grass transition and modification of near-surface hydrologic cycle

During the 19th and 20th centuries, the landscape of the North American Great Plains was rapidly modified from natural grasslands to agricultural farmlands. These changes affected much of the continent with potential impacts in grasslands equal to deforestation elsewhere. Therefore, the resulting impacts on weather and climate should be studied. In this study, a soil water balance model is applied for 3 land uses at 3 locations. These locations are representative of the east to west declin- ing precipitation gradient of the Great Plains. It was found, in McCook, Nebraska, for example, that annual total evapotranspiration for irrigated maize is 36% higher compared to natural grass. This accounts for an additional 50 million m 3 of water evapotranspired into the atmosphere from an area of 19 000 ha during the growing season. In some instances, Clay Center, Nebraska, and the vicinity evapotranspired nearly 100 million m 3 additional water (compared to a grass covered surface) from irrigated maize during the growing season. Compared to grass, irrigated maize farming elevated soil water in the soil profile, while rainfed maize lowered soil water. This study shows that intensity of the response of soil water distribution in the root zone is a function of vegetation cover and soil physical properties.

Effects of shallow water table on capillary contribution, evapotranspiration, and crop coefficient of maize and winter wheat in a semi-arid region

A lysimeter experiment was conducted during 19866—96 to study the impacts of groundwater tables on the capillary contribution, evapotranspiration, and crop coefficient of maize and winter wheat grown in a semi-arid region in loess loam soils. The depth of groundwater table was set to 0.5, 0.8, 1.0, 1.2, 1.5, 2.0, and 2.50 m, respectively. The results showed that the rate of capillary contribution from groundwater to crop root-zone was influenced mainly by the depth of the water tables. The daily variation in capillary contribution was not the same as pan evaporation; the peak was delayed when the water table was &gt;0.8 m, and the time of delay increased with the depth of water table. The crop evapotranspiration was decreased with increasing groundwater table in the early growth period and harvest period. The maximum evapotranspiration occurred at 1.2 m groundwater table in the other periods. Values of crop coefficients (K c ) were estimated based on the measured evapotranspiration (ET) and reference crop ET computed by the modified Penman method. The estimated K c was significantly different from the values computed and used in the region in the absence of groundwater table effects, and it varied markedly with groundwater tables. Relationships between the crop coefficient and the depth of groundwater table were developed using mean crop coefficients derived from multi-year data. It was found that linear model was better for the period Octobermp;mdash;February in the winter wheat growing season and June in the summer maize growing season. The polynomial model was suitable for the period March;mdahs;June in the winter wheat growing season and from July to October in the summer maize growing season.

Assessment of Climate Change and Agriculture in Spain Using Climate Models

The outputs of a general circulation model (GCM) and a regional climate model (RCM) were used as inputs for crop models to predict the impact of climate change and CO2 concentration (2×CO2 scenario) on crops and their water use in Spain. Two basins representative of irrigated agriculture were chosen to compare the phenology, aboveground biomass, yield, evapotranspiration (ET), and irrigation requirements for winter wheat (Triticum aestivum L.), winter barley (Hordeum vulgare L.), and maize (Zea mays L.) under (i) a GCM‐derived climate scenario and (ii) a RCM climate output. Spatial and temporal differences in crop performance could be enhanced by the higher resolution of the RCM. Crop phenology simulated in a 1×CO2 RCM scenario was closer to the observed phenology than the GCM‐derived one. In the 1×CO2 scenario, vegetative phenophases were shorter and reflected the higher air temperatures in the RCM. In the 2×CO2 GCM and RCM scenarios, the crop duration, ET, and irrigation requirements diminished, with no significant changes in the yield and biomass between scenarios of the same model. The higher temperatures in the RCM‐derived scenarios led to higher development rates and shorter phenophases. Nevertheless, the biomass was greater than in the GCM‐derived scenarios, especially in C3 crops. Higher temperatures in the southern areas led to a lack of vernalization in the future climate scenario, which was only detected by the RCM‐DSSAT (Decision Support System for Agrotechnology Transfer). The use of direct output of the GCM is questionable given the orography, land cover, and size of the Iberian Peninsula, and it makes the use of high‐resolution models necessary.