Modified Penman-Monteith Equation Research Articles

Assessment of irrigation water management using FAO-CROPWAT 8 in the Paliganj distributary of the Sone irrigation scheme in Bihar: a case study

ABSTRACT Around 15% of India's GDP comes from agriculture, which is the foundation of the country's economy. India's economic development and efforts to combat poverty have already benefited greatly from its agricultural sector. Sustainable management of water resources can increase agricultural productivity. The study performed in the Paliganj distributary is a part of the Sone canal system in south Bihar, India. The Paliganj distributary command area lies wholly in the Gangetic plain. The estimation of crop evapotranspiration is carried out using the modified Penman–Monteith equation for the period 1980–2015. The crop coefficient of the crops has been estimated by using a single crop coefficient and a dual crop coefficient. The crop water requirement, irrigation water requirement, and irrigation scheduling of crops grown in the study area are determined using CROPWAT 8.0 software. The results conclude that the maximum, minimum, and average values of ET0 were found to be 7.65, 1.31, and 3.86 mm/day, respectively. The crop water requirement and irrigation requirement of crops grown in kharif, rabi, hot weather, and annual seasons in the study area were lower by using dual crop coefficient.

Improving prediction accuracy of CSM-CERES-Wheat model for water and nitrogen response using a modified Penman-Monteith equation in a semi-arid region

Improving prediction accuracy of CSM-CERES-Wheat model for water and nitrogen response using a modified Penman-Monteith equation in a semi-arid region

Projecting the impact of climate change and elevated CO2 concentration on rice irrigation water requirement in China

Climate change and elevated CO2 concentrations significantly affect rice growth and water consumption. Understanding the specific impacts of climate change and elevated CO2 concentrations on rice physiological phenology, crop water demand (ETC), and irrigation water requirement (IR) is of great significance for the sustainable utilization of water resources and food security. This is particularly true in China, the world's largest rice producer. In this study, with the help of two rice phenological models, the modified Penman-Monteith equation, and the paddy water balance model, we project the changes in rice phenological period, ETC, and IR in four main rice-producing regions of China in the period 2015-2100 based on the 11 GCM outputs. The results show that the rice growing period is shortened in most rice-producing regions, except for the parts of the middle and lower reaches of the Yangtze River. Meanwhile, the trend of ETC and IR of rice varies slightly among regions in the future scenario, with almost all regions decreasing yearly except for the middle and lower reaches of the Yangtze River, where the trend is increasing. The progressively increasing atmospheric CO2 concentration has a "fertilization effect" on the crop, which can reduce the water requirements of rice. In the SSP585 scenario, the " CO2 fertilization effect" can reduce up to 8.87×108m3 of ETC and 6.94×108m3 of IR in the middle and lower reaches of the Yangtze River in the period of 2090s. This study provides beneficial references to understand the response of rice ETC and IR to future climate change and CO2 concentration elevation in China and highlights that the simulation in terms of crop irrigation must account for the "CO2 fertilization effect".

Study on the detection of water status of tomato (Solanum lycopersicum L.) by multimodal deep learning.

Water plays a very important role in the growth of tomato (Solanum lycopersicum L.), and how to detect the water status of tomato is the key to precise irrigation. The objective of this study is to detect the water status of tomato by fusing RGB, NIR and depth image information through deep learning. Five irrigation levels were set to cultivate tomatoes in different water states, with irrigation amounts of 150%, 125%, 100%, 75%, and 50% of reference evapotranspiration calculated by a modified Penman-Monteith equation, respectively. The water status of tomatoes was divided into five categories: severely irrigated deficit, slightly irrigated deficit, moderately irrigated, slightly over-irrigated, and severely over-irrigated. RGB images, depth images and NIR images of the upper part of the tomato plant were taken as data sets. The data sets were used to train and test the tomato water status detection models built with single-mode and multimodal deep learning networks, respectively. In the single-mode deep learning network, two CNNs, VGG-16 and Resnet-50, were trained on a single RGB image, a depth image, or a NIR image for a total of six cases. In the multimodal deep learning network, two or more of the RGB images, depth images and NIR images were trained with VGG-16 or Resnet-50, respectively, for a total of 20 combinations. Results showed that the accuracy of tomato water status detection based on single-mode deep learning ranged from 88.97% to 93.09%, while the accuracy of tomato water status detection based on multimodal deep learning ranged from 93.09% to 99.18%. The multimodal deep learning significantly outperformed the single-modal deep learning. The tomato water status detection model built using a multimodal deep learning network with ResNet-50 for RGB images and VGG-16 for depth and NIR images was optimal. This study provides a novel method for non-destructive detection of water status of tomato and gives a reference for precise irrigation management.

Estimation of regional farmland irrigation water requirements and water balance in Northeast China.

Agricultural water use has long accounted for more than 70% of water consumption in Northeast China. Estimating farmland irrigation water requirements and water balance is essential to ensure safe agricultural water and promote rational development and utilization of regional water resources. In this study, based on the modified Penman-Monteith equation recommended by the Food and Agriculture Organization (FAO) and Geographic Information System (GIS) technology, the net crop irrigation water requirements for four main crops in Northeast China were calculated, and the spatiotemporal distribution characteristics were also analyzed. Additionally, regional farmland irrigation water requirements were estimated, water balance in a typical year was determined, and the dominant factors affecting farmland irrigation water requirements in different regions were analyzed. From 1986 to 2020, the net irrigation water requirements for four main crops all showed the temporal trend of no significant increase and the spatial distribution characteristic of being high in the west and low in the east. The farmland irrigation water requirement decreased, and the monthly average farmland irrigation water requirement peaked in July during 2010-2019. Compared with 2010, in 2019, the irrigation water requirement per cultivated land grid cell in 20 cities increased and that in 16 cities decreased. Most cities were facing varying degrees of water shortage. Precipitation had the greatest direct effect on the farmland irrigation water requirement in different regions. These results quantify the farmland irrigation water requirement and water balance in Northeast China, and provide a reference for water resources and related environmental governance.

Variable-Rate Fertigation Control for Hydroponics Using an On-The-Go Transpiration Monitoring System

HighlightsAn on-the-go monitoring system was developed for assessing spatial and temporal variations in crop growth.The system was used to realize real-time transpiration estimation for lettuce growth.A variable-rate fertigation system was developed based on on-the-go transpiration monitoring.The variable-rate fertigation system was validated in controlled-environment cultivation of hydroponic lettuce.Abstract. Variable-rate fertigation, as a key component of hydroponic cultivation, ensures the sustainable use of water. However, this fertigation method is challenging to implement because of the difficulty associated with assessing the water needs of plants, which vary with plant growth stage and environmental conditions. In this study, an on-the-go crop growth monitoring system was developed to measure the canopy cover of lettuce plants on a daily basis. The canopy cover during the vegetative growth period could be estimated with an accuracy of 98.5% ±1.7% compared to manual segmentation. A modified Penman-Monteith equation was used to predict the transpiration rate of the lettuce plants. In the first cultivation test, the transpiration rate model exhibited a highly linear relationship with a slope of 0.91, coefficient of determination of >0.9, and standard error of regression of <0.51, compared to the directly measured transpiration rates. In the second cultivation test, an automated system that could variably control the fertigation volume was incorporated with the on-the-go system for monitoring the lettuce transpiration in real time. The proposed system could realize on-the-go mapping of the spatial distribution of the canopy cover of the growing lettuce plants, and the variable fertigation responded to the daily water consumption of the plants with an error of only 7.3%. Moreover, the stable canopy cover growth indicated the higher efficiency of variable fertigation compared to timer-based fertigation systems. The proposed variable-rate fertigation system is a promising and practical approach to implement hydroponics with reduced water and energy consumption. Keywords: Canopy cover, Hydroponic, On-the-go monitoring, Transpiration, Variable fertigation.

Modelling Canopy Actual Transpiration in the Boreal Forest with Reduced Error Propagation

The authors have developed a scaling approach to aggregate tree sap flux with reduced error propagation in modeled estimates of actual transpiration () of three boreal species. The approach covers three scales: tree point, single tree trunk, and plot scale. Throughout the development of this approach the error propagated from one scale to the next was reduced by analyzing the main sources of error and exploring how some field and lab techniques, and mathematical modeling can potentially reduce the error on measured or estimated parameters. Field measurements of tree sap flux at the tree point scale are used to obtain canopy transpiration estimates at the plot scale in combination with allometric correlations of sapwood depth (measured microscopically and scaled to plots), sapwood area, and leaf area index. We compared the final estimates to actual evapotranspiration and actual transpiration calculated with the Penman–Monteith equation, and the modified Penman–Monteith equation, respectively, at the plot scale. The scaled canopy transpiration represented a significant fraction of the forest evapotranspiration, which was always greater than 70%. To understand climate change impacts in forested areas, more accurate actual transpiration estimates are necessary. We suggest our model as a suitable approach to obtain reliable estimates in forested areas with low tree diversity.

Coupled impact of decadal precipitation and evapotranspiration on peatland degradation in the Zoige basin, China

ABSTRACTThis study quantified the temporal properties of precipitation and actual evapotranspiration (ETa) in the Zoige basin of the Yellow River source region, China during the 1967–2011 period, as well as their influence to the area reduction of peatland. We extracted areas of different land-use and land-cover (LULC) types and obtained daily precipitation data. Then, we calculated annual precipitation (AP) and specific cumulative precipitation (SCP), which is the sum of precipitation to the date when an image was taken, and showed that the peatland areas were strongly affected by SCP. Using a modified Penman–Monteith equation, we calculated ETa for each LULC type and the area-weighted ones to show that the area-weighted total ETa was mainly contributed from grassland and peatland, which was between 450 and 550 mm. Temporal trends of the ratio of SCP to evapotranspiration showed that precipitation was generally greater than evapotranspiration rate not only during the summer but also over the 1967–2011 period. This trend failed to completely explain the continuous decrease of peatland area in the Zoige basin in decades. The draining effect of artificial ditches and natural gullies might play an additional role in causing peatland degradation but requires further process-based studies.

A modified FAO evapotranspiration model for refined water budget analysis for Green Roof systems

Abstract Many metropolitan cities around the world are promoting the implementation of Green Roofs (GR) as effective stormwater management systems to mitigate adverse effects of urbanization. Hourly water balance calculations are necessary for accurate prediction of the antecedent soil moisture conditions of a GR, a key parameter for rainfall-runoff modeling. However, existing evapotranspiration (ET) models have substantial errors for hourly ET predictions over a range of moisture conditions to accurately assess the hydrological performance of the GRs during storm events. Therefore, a modified Penman-Monteith equation was developed to provide improved prediction of hourly ET specifically for GR applications. ET rates predicted by the modified and established Penmen-Monteith equations were compared with ET data collected from a GR lysimeter over the summer and fall of 2016. The proposed modified Penman-Montieth equation eliminates the water availability factor ( θ i θ fc ) included in the equation’s advection term and separates the effect of plant coefficients on radiation/energy and advection terms. The modified equation was found to improve ET estimates by 8–9% (RMSE: 0.535–0.493, MAD: 0.375–0.343), under non-water limited conditions. For water-limited conditions, errors were significantly reduced. RMSE improved by 37% (0.703–0.440) and MAD was improved by 31% (0.429–0.294).

Modified Penman–Monteith equation for monitoring evapotranspiration of wheat crop: Relationship between the surface resistance and remotely sensed stress index

Evapotranspiration (ET) plays an essential role for detecting plant water status, estimating crop water needs and optimising irrigation management. Accurate estimates of ET at field scale are therefore critical. The present paper investigates a remote sensing and modelling coupled approach for monitoring actual ET of irrigated wheat crops in the semi-arid region of Tensift Al Haouz (Morocco). The ET modelling is based on a modified Penman–Monteith equation obtained by introducing a simple empirical relationship between surface resistance (rc) and a stress index (SI). SI is estimated from Landsat-derived land surface temperature (LST) combined with the LST endmembers (in wet and dry conditions) simulated by a surface energy balance model driven by meteorological forcing and Landsat-derived fractional vegetation cover. The proposed model is first calibrated using eddy covariance measurements of ET during one growing season (2015–2016) over an experimental flood-irrigated wheat field located within the irrigated perimeter named R3. It is then validated during the same growing season over another drip-irrigated wheat field located in the same perimeter. Next, the proposed ET model is implemented over a 10 × 10 km2 area in R3 using a time series of Landsat-7/8 reflectance and LST data. The comparison between modelled and measured ET fluxes indicates that the model works well. The Root Mean Square Error (RMSE) values over drip and flood sites were 13 and 12 W m−2, respectively. The proposed approach has a great potential for detecting crop water stress and estimating crop water requirements over large areas along the agricultural season.

Estimation of regional irrigation water requirement and water supply risk in the arid region of Northwestern China 1989–2010

Water use in agricultural sector shares more than 90% of the total water withdrawal in the arid region of Northwestern China (hereafter, ARNWC). Irrigation water demand is therefore essential to the water resources allocation to economy and natural ecosystems in the highly water deficit region. In this study, we analyzed the spatial and temporal variations of irrigation water demand as well as crop water requirement by combining the modified Penman–Monteith equation recommended by FAO and GIS technology. Crop and irrigation water requirements for 5 main crops, including wheat, corn, cotton, oilseed and sugar beet, from 1989 to 2010 were calculated and the spatio-temporal variations were analyzed. The results suggested that the demand of irrigation water in the ARNWC showed increasing trend during the past two decades, which mainly caused by fast increase in cotton cultivation areas, because irrigation water requirement for cotton was much larger than the other crops. The changes in cotton growing area significantly affected the spatial pattern of water demand. A total of 44.2 billion m3 water was withdrawn for irrigation in year 2010. Larger amount of water was consumed for crops in Northern Xinjiang and Tarim River Basin than Qilian-Hexi region. Irrigation water requirement reaches its maximum in July and August. It is revealed that the critical period for water supply is during April and May through comparing the monthly irrigation water requirement with water availability, i.e. river discharge. Even though the annual water resources are much larger than the requirement, but for some basins, there is severe physical water shortage during the critical water use period in April and May. The water resource supply is expected to be facing more difficulties in future.

Modelagem do balanço hídrico em microbacia cultivada com plantio comercial de Eucalyptus grandis x urophylla no leste de Minas Gerais, Brasil

No Brasil, o número de plantios comerciais de espécies florestais de crescimento rápido vem aumentando consideravelmente nas últimas décadas. Contudo, os possíveis impactos desse aumento sobre os recursos hídricos ainda são desconhecidos. O objetivo deste estudo foi realizar a modelagem do balanço hídrico em uma microbacia cultivada com plantio comercial de híbridos de Eucalyptus grandis x urophylla no Leste de Minas Gerais, Brasil. O trabalho foi realizado em uma microbacia experimental com área total de 40 ha durante o ano hidrológico, compreendido entre outubro de 2009 e setembro de 2010. A principal entrada de água no sistema via precipitação pluvial e os dados meteorológicos foram coletados por uma estação automática instalada no interior da área de estudo. As principais saídas foram estimadas pela equação modificada de Penman-Monteith e pela mensuração do deflúvio gerado na saída da bacia. Do total precipitado (1.123 mm) durante o ano, 9% (106 mm) retornaram à atmosfera via interceptação, 66% (740 mm) via transpiração da cultura e 11% (119 mm) pela evaporação do solo, que somados corresponderam à aproximadamente 86% da entrada de água na microbacia. Do deflúvio total gerado (~147 mm), apenas 4,29 mm (0,4%) foram convertidos em escoamento superficial. Pode-se concluir que durante o ano hidrológico monitorado o balanço entre as principais entradas e as saídas de água na microbacia foi equalizado, não comprometendo a disponibilidade hídrica natural no curso d'água.

Simulating evapotranspiration of Qinghai spruce ( Picea crassifolia) forest in the Qilian Mountains, northwestern China

Qinghai spruce (Picea crassifolia) forests play an important role in regulating the regional water balance of the Qilian Mountains in northwestern China. The objective of this study was to estimate evapotranspiration (ET) of the Qinghai spruce forest in the middle section of the Qilian Mountains. A modified Penman–Monteith equation was used to simulate two sub-components of ET: canopy transpiration (Et) and soil evaporation (Es). The third sub-component of ET, canopy interception evaporation (Ei), was directly measured. The results show that the total ET of the Qinghai spruce forest was 313.6 mm during the 2008 growing season (from May 1 to September 30). Ei, Et, and Es were 100.9 mm, 160.8 mm and 51.9 mm, and accounted for 32.2%, 51.3% and 16.5% of the total ET, respectively. The total modeled ET during the 2008 growing season is acceptably consistent with the directly measured ET (298.2 mm) by eddy covariance system. The consistency implies that the modified Penman–Monteith equation is an effective method to estimate ET using conventional meteorological data with additional measurements of net radiation, and the method can thus be applied to similar situations where reliable direct measurements are not practical.

Landscape Coefficients for Single- and Mixed-species Landscapes

Urban landscape irrigation is becoming increasingly important from a resource management point of view. Significant water use savings may be achieved if landscape irrigation is based on reference evapotranspiration (ETo). This study measured landscape crop coefficients (KL) for landscapes that are comprised of different vegetation types and irrigation water quality differences affecting KL. The KL was determined from the ratio of actual evapotranspiration to the ETo calculated from the modified Penman-Monteith equation. Irrigation quantity was based on 100% replacement of ETo. The KL values were determined for the following landscape vegetation on a fine sandy loam: St. Augustinegrass [Stenotaphrum secundatum (Walt.) Kuntze.], a single shumard red oak (Quercus shumardii Buckl.), St. Augustinegrass plus red oak, native grasses [Muhlenbergia capillaries (Lam.) Trin. and Schizachyrium scoparium (Michx.) Nash], and native grasses plus Red Oak in both College Station (CS) and San Antonio (SA), TX. Soil was systematically placed into lysimeters containing a drainage system and soil moisture probes. Lysimeters (1136 L) were placed in-ground in a randomized complete block design with three blocks. Soil moisture measurements were made at 0- to 20-, 20- to 40-, and 40- to 60-cm depths. The KL was determined after a rainfall or irrigation event for periods of 2 to 5 days. During the combined growing seasons of 2007 and 2008, KL in SA increased from early, to mid, to late season. In CS, the KL was unaffected by plant treatment or season. The St. Augustinegrass treatment KL seasonally ranged from 0.45 to 0.62 in SA. In CS, soil sodium accumulation caused decreased KL. These results of KL for mixed-species landscapes on non-sodic sites trend toward seasonal values of 0.5 to 0.7 for irrigation decisions in southern Texas. Landscape coefficients can be used as a tool in irrigation decision-making, which could contribute to water savings in amenity landscapes.

Perceptive research on wheat evapotranspiration in Pakistan

AbstractThe Mehran model is developed based on the FAO‐56 modified Penman–Monteith equation for computing reference and crop evapotranspiration with dual crop coefficients. Evapotranspiration (ET) of wheat in three field experiments with management‐allowed depletion (MAD) of 45, 55 and 65% was observed through gypsum block readings and a drainage lysimeter. The seasonal crop ETs were observed as 363, 359 and 332 mm, and were computed as 383, 369 and 355 mm. The corresponding water use efficiencies (WUEs) were ascertained as 14.1, 15.0 and 13.4 kg (ha mm)−1. The highest crop WUE was achieved with MAD at 55%; therefore, this research is more focused upon in the paper. The model relatively overestimated seasonal ET by 2.8%. Weekly root length and daily soil‐moisture measurements revealed that wheat extracts most of its moisture from the 0–50 cm soil profile. When practising either scientific or traditional irrigation scheduling in the country, a seasonal water amount of 370 mm is suggested for wheat to achieve optimum yield and WUE.Statistical analysis (R2 = 0.90, T = 2.09, and F = 999) showed good correlation between computed and actual seasonal ETs of crop with an experimental MAD of 55%. The Mehran model is found to be quite versatile and can be successfully used as a decision support system (DSS) for irrigation scheduling of wheat in Pakistan. Copyright © 2008 John Wiley & Sons, Ltd.

Remote sensing to estimate ET-fluxes and the performance of an irrigation district in southern Italy

Satellite remote sensed data on canopy biophysical properties, ground data and agro-meteorological information were combined to estimate evapotranspiration (ET) fluxes of orange orchards using a modified Penman–Monteith equation. The study was carried out during the irrigation season 2004 in an irrigation district , cover for about 95% with orange groves, of 1550 ha located in eastern Sicily (Italy). The spatial pattern in ET-fluxes have been analysed using IKONOS high-resolution satellite and hyper-spectral ground data acquired and processed for the study-area. The remote estimates of ET-fluxes varied between 1.3 and 5.7 mm/day, with a daily average value of about 4.2 mm, showing a good agreement with crop ET values determined as residual of soil water balance of selected ground control sites. Crop coefficient estimates ranged between 0.22 and 1.08 showing positive correlations with percentages of ground cover ( C g ) increasing from 30 to 80% ground shading and with LAI values. By comparing ET estimates with water volumes supplied in each sub-district of the study-area, the performance indicator “IP” was evaluated, allowing to rank the conditions of un-fulfilment of crop water requirements by public and private water distribution systems . Generally, out of 29 sub-districts, 14 had “IP” values less than 50%, revealing a sub-optimal water supply for the study-area.

Measuring and predicting evapotranspiration in an insect-proof screenhouse

Abstract This study addresses the current lack of accurate measurements and predictions of evapotranspiration below modern screening materials. Energy balance components, microclimatic and physiological parameters were measured in a 50-mesh insect-proof screenhouse cultivated with sweet pepper. A lysimeter-calibrated sap flow system and an eddy covariance system installed inside the screenhouse measured canopy transpiration and evapotranspiration, respectively. Sap flow (SF) and eddy covariance (EC) data showed good agreement ( R 2 = 0.95) and although EC exceeded SF by 9%, this was less than the standard error of inter-specific sap flow, supporting previous findings that mid-day soil evaporation was negligible. Screenhouse bulk resistance was inversely correlated with external wind speed and simple mass balance estimations gave reasonable values of evapotranspiration on a daily basis. A one-dimensional screenhouse model was derived, based on a modified Penman–Monteith equation incorporating an additional boundary layer resistance. The model, along with a greenhouse model, was used to calculate evapotranspiration of the screenhouse crop. Half hourly predictions of both models yielded good agreement with measured SF ( R 2 = 0.94–0.95). Daily crop water use, predicted with the greenhouse model was in better agreement with measurements than that predicted by the screenhouse model. Measured and predicted transpiration rates inside the screenhouse were approximately 1.8–2.1 mm day −1 during the most active stage of growth, while those simulated for a similar crop grown outside would be 4.5–5.3 mm day −1 on average. Model sensitivity analysis showed that reduced radiation and wind speed and modified vapour pressure deficit were the main factors influencing transpiration. Calculations of the decoupling factor Ω suggest that the screenhouse evaporative climate is predominantly “decoupled”.

ENVIRONMENTAL CONTROL SYSTEMS FOR MIST PROPAGATION OF CUTTINGS

ENVIRONMENTAL CONTROL SYSTEMS FOR MIST PROPAGATION OF CUTTINGS

Accuracy of the neutron probe for measuring changes in soil water storage under potatoes

The neutron probe (NP) is used widely to measure changes in soil water storage in research and more recently to aid irrigation scheduling. Its accuracy is rarely questioned and most of the relationships between soil water changes and productivity are based on its use. A field experiment was conducted at Cambridge University Farm in 1999 to address whether the NP could accurately measure changes in soil water content (SWC) under irrigation or substantial rain (>10 mm). The experiment was a replicated split-plot design with four irrigation treatments allocated to the main plots, and surface profile (ridge, flat) and crop (potato cv. Saturna, bare soil) treatments allocated to the subplots. The mean results from four NP access tubes per plot installed to measure soil moisture deficit (SMD) across the row-width were analysed. The NP was inconsistent in measuring known irrigation or rainfall input. In relatively dry soil (SMD>40 mm), the NP generally measured 93 to 110% of 18 mm of irrigation within 4 h of irrigation. The NP recorded much less water applied as irrigation in wetter soil, and often only 40 to 70% of the applied irrigation (18 or 36 mm) was measured. There were occasions when the NP did not measure all the water input even when the SMDs before irrigation were greater than the water subsequently applied. Some of the ‘missing’ water might be attributed to drainage, however, results from an additional experiment using an open-topped tank of soil showed that the NP was unable to detect all the water added to the soil, particularly where the water was largely confined close to the soil surface. Replicated measurements of the change in SMD in the field experiment were precise for a given event and treatment (mean S.E. = 1·3 mm) but were not accurate when compared against the input measured in rain gauges. It was concluded, that the NP could not be used reliably to measure changes in soil water storage after irrigation or substantial rain. For periods when there were minimal inputs of water, there was a closer correlation between changes in SMD measured by the NP and those predicted by a modified Penman–Monteith equation than after substantial inputs of water. However, for predicted changes in SMD of c. 20 mm, there was a range of c. ±5 mm in the changes in SMD measured by the neutron probe.The value of the NP for monitoring SMDs where there is irrigation, or substantial rain, must be seriously doubted. Consequently, its limitations for scheduling irrigation, testing models or quantifying the effects of treatments on crop water use in potatoes must be appreciated, especially where the areal sampling limitations of single access tubes positioned only in the ridge centre have not been addressed.

Measurement and Calculation of Total Plant Water Use of Citrus

This preliminary study serves to parameterize a modified Penman–Monteith equation for young citrus trees. Two-year-old citrus trees (`Hamlin' grafted on Carrizo citrange) were planted individually into 1500-L (1.5-m-diameter) plastic pots in late Oct. 1997. Pots were placed upon industrial scales with a maximum weight capacity of 2270 ± 0.05 kg. Continuous weight measurements were made every minute and average half-hourly values recorded. An automated weather station was located within 10 m of the pots and half hourly values of temperature, humidity, wind speed, rainfall, and net radiation were recorded. Meteorological data, coupled with diurnal measurements of stomatal conductance taken periodically throughout the experimental period and leaf area for each tree allow calcuatiion of total plant water use using a modified Penman–Monteith equation. These calculation may be cross-checked using weight change measurements. Such model parameterization for citrus will aid irrigation management of citrus in the field as it provides a link between the physiological and meteorological aspects which drive plant water use.