A comparison of the Priestley-Taylor and Penman methods for estimating reference crop evapotranspiration in tropical countries

An equation for Potential Evaporation (PE) proposed by Priestley and Taylor in 1972 has fewer data requirements than the well established Penman Potential Transpiration (Et) equation. From their definitions, PE and Et values should both provide acceptable estimates of Reference Crop Evapotranspiration (ETo), as defined by Doorenbos and Pruitt. Analysis of mean monthly climatic data from 30 tropical stations, widely spread within the latitude zone 25°N to 25°S, showed that PE and Et estimates agreed closely when monthly rainfall exceeded monthly Et. The minimum data requirements for the Priestley-Taylor equation are daily net radiation and mean air temperature. The Penman equation additionally requires daily data for humidity and run of wind. As reliable field net radiometers become more widely available, the Priestley-Taylor PE equation offers a satisfactory alternative to the Penman Et equation for estimating ETo in humid tropical climates.     

A discussion on and alternative to the Penman-Monteith equation

The Penman-Monteith (PMe) equation that estimates evaporation from leaf/canopy surfaces is based on a few approximations. Several authors discussed ensuing errors and suggested improvements. This paper reminds those discussions which ended in the early nineties. It compares linearized PMe− with non-linear iterative solutions and illustrates resulting deviations. It differentiates between deviations for daily and hourly evaporation rate estimates. The latter are found to be higher. It also demonstrates deviations obtained at two different altitudes above sea level. Considering present tendencies to refine evaporation estimates for practical purposes and making use of easily available methods for solving non-linear equations this paper offers a new method to estimate evaporation.In a first step, a simple algebraic term, the surface temperature control sum, is introduced to find approximate differences between air and evaporating surface (leaf, canopy) temperatures. It suggests to concentrate research on the r_s/r_a ratio. A new formula is derived for estimating leaf/canopy surface temperatures for non-water stressed plants.In a second step, the estimates of temperature differences are used to calculate evaporation estimates. This two-step approximation leads to appreciably smaller errors as compared to the PMe-solution over the full range of input parameters of agro-meteorological relevance. It is, however, less accurate than some of methods proposed in literature. The method is meant for practical application in agricultural water management.     

Penman-Monteith公式与Penman修正式在计算ET0中的比较研究

利用1961-2001年关中中部地区3个气象站的月气象资料,分别用Penman-Monteith公式和Penman修正式计算3站的ET0,并对计算的结果进行了比较.结果表明,Penman-Monteith公式计算的ET0年值大于Penman修正式计算的ET0的年值,且两种方法逐年的计算结果比较稳定.绝对误差△ET0的变化为-138～44 mm;相对误差变化为0.17%～13.90%.ET0多年月均值在全年各月变化较大,绝对误差△ET0在4-7月大于0,其他月份都小于0;相对误差1、10、11和12月相对较大,而在其他月份相对较小.导致差异的原因在于两种公式采用了不同的辐射项和空气动力项计算公式和参数,其中春、夏两季空气动力学项的不同是引起结果差异的主要原因,而在秋、冬两季辐射项不同是引起结果差异的主要原因.两种公式计算的ET0具有显著的线性相关性.     

A quantitative framework to analyse cooperation between rural households

Different types of cooperative agreements between smallholders continue to play an important role in rural areas in developing countries. While some empirical studies examine the conditions catalysing the successful formation of cooperatives, quantifications of the net benefits, i.e., difference between revenues and costs, of cooperation and how farmers divide these net benefits are scarce. Therefore, we develop a quantitative framework to analyse and allocate net benefits in a cooperative production agreement. The framework allows for cooperative exchange of several types of resources and the production of multiple products.Linear programming provides insight into optimal production levels, both for individual and cooperating farmers, and gives optimal revenue levels. A transaction cost function is used to account for costs of cooperation, such as meeting costs, moral hazard and free ridership of labour use and the risks of farmers defaulting from the agreement. Transaction costs are likely to increase with the number of households participating, the total cropping area and the heterogeneity of resources of the cooperating farmers. Therefore, we introduce a measure of heterogeneity in the resources for each cooperative. Finally, cooperative game theory is used to generate fair divisions of the net benefits in a cooperative.This framework may be used to give additional explanations to the findings in empirical studies on cooperatives. We illustrate this with an empirical example from northern Nigeria. It is found that cooperation between farmers sharing complementary resources gives the highest revenues. Next, we illustrate the effects of two different transaction cost functions. For reasonable assumptions on these functions, cooperation remains economically attractive. Nevertheless, larger and more diverse coalitions are not always the most beneficial, while the returns in some small coalitions are negative, possibly impeding the formation of cooperatives in some locations.     

A conceptual approach to the modelling of herbage intake by hill sheep

A modelling approach is examined as a method of investigating alternative management strategies for improved systems of hill sheep production. In order to take account of the wide range in the quality of hill herbage and the ability of the hill sheep to select preferred components, herbage has been conceptually classified in terms of its digestibility; the digestibility classes are then grazed selectively by sheep. The model deals with herbage growth and deterioration, diet selection and the maintenance and liveweight change of wether sheep. Results produced by the model, in which wethers grazed Agrostis-Festuca at two stocking rates, are discussed.     

A hard systems approach to the management of quality in apple distribution

The management of quality in the distribution of produce which exhibits significant and inevitable deterioration with time and event is a neglected but essential component of physical distribution management. This paper concentrates on horticultural produce and, in particular, on apples.Horticultural distribution is described as a system and systems thinking is employed to develop a ‘hard systems’ methodology for the management of quality. A management system is defined in terms of quality-price, quality-cost and quality-happening relations. An example from apple distribution demonstrates the methodology and is used to emphasize the central importance of quantitative models of price, cost, quality and of their interactions with time and happening.     

A Markov chain approach to crop yield forecasting

This study proposes a methodology for forecasting crop yields at intermediate times in the growing season using Markov chain theory. A Markov chain is constructed, based on historical data, to provide forecast distributions of crop yield for various crop and soil moisture condition classes at selected times prior to harvest. Expected yield and the associated standard error are obtained for each condition class. The methodology is compared to a regression approach in which the independent variables are the various crop and soil moisture conditions. The Markov chain approach requires less stringent assumptions and provides more information than the regression approach. However, the potential loss of precision in the forecast using this approach requires separate evaluation for each application. A data base created by the CERES-Maize model, which simulates the growth and development of a corn crop, is used to demonstrate the development of the forecast yield distributions using the Markov chain approach.     

A multi-objective programming approach to feed ration balancing and nutrient management

This paper examines the potential to use multiple objective programming to reduce nutrient excretion from dairy cows through incorporation of nutrient excretion functions into a ration formulation framework. In a typical ration formulation model, a ration is formulated to minimize cost while providing sufficient nutrients to meet the needs of the animal type being fed. To reduce the nutrient loading, rations can be formulated to minimize cost, and nitrogen and phosphorus excretion using multiple objective programming. Rations were initially formulated to minimize cost, nitrogen excretion and phosphorus excretion. Compromise programming was then utilized to examine the impacts on ration formulation of combining the three individual objectives. The multiple objective ration formulation reduced phosphorus excretion by 5% and marginally reduced nitrogen excretion with a small increase in ration cost compared to the single objective minimum cost ration. Multiple objective programming does have the potential to reduce nutrient excretion.     

企业信息系统实施的系统工程方法

企业信息系统的实施本质上是一个系统工程,为此从系统工程角度分析了系统实施的组成要素及其关系,提出了企业信息系统实施的系统工程方法。     

常见电力电缆故障的处理方法探讨

电力电缆作为整个电力系统的重要组成部分,一旦发生故障将直接影响着整个电力系统的安全运行。文章分析了电力电缆故障的原因及分类,探讨了电力电缆的故障实行的检测方法,保证了电力供应畅通、保证供电可靠性。     

A new approach to water management of tropical peatlands: a case study from Malaysia

The use of peatlands in the humid tropics requires drainage to remove excess rainfall. The design principles for the drainage systems currently being implemented on peatlands are the same as for mineral soils. The objective of such systems is the timely removal of excess rainfall by surface runoff. For peatlands, with their different soil-hydraulic characteristics, these systems have resulted in poor watertable control and high rates of irreversible subsidence. Concerns about this rate of subsidence and the level of sustainability of the present land use have prompted a study to develop a new water management system. This new system includes a shift from a drainage system that focuses on discharge of excess water towards a system that combines drainage and water conservation. In the new two-step design, the drain spacing and corresponding drain discharges are obtained with a steady-state approach. These outputs are used to calculate the capacity of the drains, including control structures, using an unsteady-state approach. The new system results in a shallower but more narrowly spaced drainage system and maintains a more constant but relatively high watertable and reduces subsidence. It should be remembered however, that even with the improved water management, subsidence cannot be arrested; it is the price one has to pay for the use of tropical peatlands.     

A participatory, farming systems approach to improving Bali cattle production in the smallholder crop-livestock systems of Eastern Indonesia

Bali cattle (Bos javanicus) account for about one quarter of the total cattle population in Indonesia and are particularly important in the smallholder farming enterprises of the eastern islands. The population of Bali cattle is declining in most areas of Eastern Indonesia because demand for beef cattle exceeds the local capacity to supply these animals. Indonesian agencies recognise that new strategies are required to improve the productivity of Bali cattle and to address major constraints relating to animal husbandry and nutrition. To date, the adoption of cattle improvement technologies has been historically slow in Indonesia, as is the case elsewhere.This paper reports on key findings from a long-term study conducted between 2001 and 2009 with smallholder households from six villages in South Sulawesi and Central Lombok, to develop and test an approach for evaluating and increasing the adoption of cattle and forage improvement technologies. The approach is based on the principles of farming systems and participatory research and involved four main steps; (1) benchmarking the current farming system; (2) identifying constraints to cattle production and strategies to address them; (3) desktop modelling of the production and economic impacts of selected strategies; and (4) on-farm testing of the most promising strategies with 30 participant smallholder households.The approach was found to be successful based on: (1) sustained adoption of a package of best-bet technologies by the 30 participating households; (2) evidence of positive production, social and economic impacts; and (3) significant diffusion of the cattle improvement technologies to other households in the project regions.     

抓住机遇转变方式推进农机推广——丁翔文在全国农机推广站站长会议上的讲话(摘要)

今年是我国十一五的最后一年,也是我国农业生产遭受自然灾害较多的一年.沧海横流,方显出英雄本色,在这一年中,农业机械化和农机推广工作不仅在农业生产的抗旱、抗低温抢种、三夏的抢收抢种、三秋的秋收秋种等一系列生产中发挥了机械化威力和生力军的作用,保证了农业生产形势的基本稳定;而且在抗洪排涝抢险等抗灾救灾中也发挥了积极的作用,凸显了机械化快速、高效、有力的保障作用和农机化队伍敢打硬仗的作风.     

Technical and economic criteria in agricultural production: A case for a systems approach to farm decision-making in the Pakistan Punjab

This paper defines the criteria of ‘technical’ and ‘economic’ efficiencies. The relationship between these criteria is identified, and its implications are discussed. These two sets of criteria do diverge, with ramifications both at the farm and the national levels. An illustration of their relationship at the farm level is provided by examining the nature of water allocation to crops in the Punjab of Pakistan. The divergences that exist between ‘technical’ and ‘economic’ efficiencies at both farm and national levels are highlighted by considering the process of mechanisation in the Punjab in some detail. Finally, it is pointed out how a ‘systems approach’ can accomodate both the technical and economic criteria and their interaction.     

A simple approach for regulating saline groundwater inflows to natural streams in irrigated agricultural areas

Stream salinity management is of prime importance for ensuring environmental sustainability of rivers, streams, lakes and other water bodies. Saline groundwater inflows are the main cause of deteriorating quality of stream flows; especially during periods of low flows. The Murray-Darling Basin is a good example for reducing groundwater recharge from irrigated agricultural areas with successful implementation of land and water management plans (LWMPs). In most cases, these plans help minimize the influence of saline groundwater flows on the stream salinity, as a preventive measure. As a remedial measure, this paper introduces a simple but an innovative approach for regulating saline groundwater inflows to natural streams in irrigated agricultural areas. The approach uses the flap gate with an automatic lowering and raising mechanism. During low stream flow, this gate is kept in a raised position. Depending upon the upstream inflows, the ponding situation will develop hydraulic gradient away from the stream; groundwater inflows to the creek will become minimal. During high stream flows, the gate is kept in a lower position to let the diluted water flows through the stream uninterrupted. To install, operate and manage such gates on multiple sites along the stream(s), the individual control panels can be joined into a central control station via telemetric link up. This approach, which holds a key for successfully regulating groundwater induced salinity to natural streams, should be considered in conjunction with other LWMPs to improve stream salinity in an irrigated agricultural area.     

A spatial reasoning approach to estimating paddy rice water demand in Hwanghaenam-do,North Korea

The primary objective of this study was to estimate the agricultural water demand of paddy fields in Hwanghaenam-do, North Korea. Three Landsat TM images, GIS data including digital elevation maps, a Thiessen network and administration maps of North Korea, and meteorological data were synthesized for this study. In order to estimate water demand for agricultural use, the FAO Blaney–Criddle method and 10-day crop coefficients of the northern areas of South Korea were used. To classify the Landsat images, supervised and unsupervised classification methods were conducted. Topographical constraints based on paddy rice growing conditions, which are under 7% slope and 200 m above sea level, were taken into account. The results showed an annual net water demand of 611.7 mm/year (916.4 Mt/year) is required for the 150,079 ha of paddy fields and the average gross water demand and design water demand for paddy rice were estimated to be 939.6 mm/year (1408 Mt/year) and 1131.97 mm/year (1695.1 Mt/year), respectively.     

System approach to drainage,irrigation and soil conservation related to water quality problems

The system approach and combination of simulation and optimization models to the problems of water quality in public water supply basins influenced by agricultural activities, drainage, irrigation and soil conservation are presented. The methodological aspects of the problem are analysed.     

An approach to multi-criterion evaluation of irrigation system performance

Irrigation systems aim to meet multiple objectives and performance must therefore be assessed using quantifiable measures for each. It therefore becomes an extremely difficult task to capture the valuation of all of them simultaneously and to mentally process the trade-offs between them in order to arrive at an overall impression of system performance. There is a need for a methodology which provides a systematic approach to comparing and combining the components of overall performance.Multi-Attribute Utility Theory offers an attractive approach to assessing performance of irrigation systems in the form of a utility function which reflects the strength of preferences and trade-offs between individual performance criteria. The method is described and demonstrated on the basis of a case study of irrigation in Sudan.     

An entropy approach to spatial disaggregation of agricultural production

While agricultural production statistics are reported on a geopolitical – often national – basis we often need to know the status of production or productivity within specific sub-regions, watersheds, or agroecological zones. Such re-aggregations are typically made using expert judgments or simple area-weighting rules. We describe a new, entropy-based approach to making spatially disaggregated assessments of the distribution of crop production. Using this approach, tabular crop production statistics are blended judiciously with an array of other secondary data to assess the production of specific crops within individual ‘pixels’ – typically 25–100 square kilometers in size. The information utilized includes crop production statistics, farming system characteristics, satellite-derived land cover data, biophysical crop suitability assessments, and population density. An application is presented in which Brazilian state level production statistics are used to generate pixel level crop production data for eight crops. To validate the spatial allocation we aggregated the pixel estimates to obtain synthetic estimates of municipality level production in Brazil, and compared those estimates with actual municipality statistics. The approach produced extremely promising results. We then examined the robustness of these results compared to short-cut approaches to allocating crop production statistics and showed that, while computationally intensive, the cross-entropy method does provide more reliable estimates of crop production patterns.