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991.
Sharp-crested weirs are the simplest form of over-flow spillway that commonly used to determine the flow rate in hydraulic
laboratories, industry and irrigation systems, where highly accurate discharge measurements are needed. In this study, the
experimental upper and lower nappe profiles in rectangular sharp-crested weirs are fitted by quadratic and cubic equations,
respectively. In addition, free-vortex theory is used to simulate flow over this kind of weirs and determine discharge coefficient.
Physical models of sharp-crested weirs with various widths and heights were considered. The proposed method agrees well with
the experimental observations. Also, the experimental data indicate that the suggested equation presents reasonable results
for the range of 0 < h/P < 9. 相似文献
992.
José AlvesJr Marcos V. Folegatti Larry R. Parsons Wije Bandaranayake Claudio R. da Silva Tonny J. A. da Silva Luís F. S. M. Campeche 《Irrigation Science》2007,25(4):419-428
The expansion of permanent trickle irrigation systems in Sao Paulo (Brazil) citrus has changed the focus of irrigation scheduling
from determining irrigation timing to quantifying irrigation amounts. The water requirements of citrus orchards are difficult
to estimate, since they are influenced by heterogeneous factors such as age, planting density and irrigation system. In this
study, we estimated the water requirements of young ‘Tahiti’ lime orchards, considering the independent contributions from
soil evaporation and crop transpiration by splitting the crop coefficient (Kc = ETc/ETo) into two separate coefficients; Ke,
a soil evaporation coefficient and Kcb, a crop transpiration coefficient. Hence, the water requirement in young ‘Tahiti’ lime
(ETy) is ETy = (Ke + Kcb) · ETo, where ETo is the reference crop evapotranspiration. Mature tree water requirement (ETm) is ETm = Kcb · ETo, assuming no soil water evaporation. Two lysimeters were used; one was 1.6 m in diameter and 0.7 m deep, and
the other was 2.7 m in diameter and 0.8-m deep. The first one was used to calculate evaporation and the second one was used
for transpiration. ETo was estimated by the Penman–Monteith method (FAO-56). The measurements were conducted during a period
between August 2002 and April 2005 in Piracicaba, Sao Paulo state, Brazil. The lysimeters were installed at the center of
a 1.0-ha plot planted with ‘Tahiti’ lime trees grafted on ‘Swingle’ citrumelo rootstock. The trees were 1-year old at planting,
spaced 7 × 4 m, and were irrigated by a drip irrigation system. During the study period, Kc varied between 0.6 and 1.22, and
Kcb varied between 0.4 and 1.0. The results suggested that for young lime trees, the volume of water per tree calculated by
Ke + Kcb is about 80% higher than the volume calculated using Kc. For mature trees, the volume of water per tree calculated
using just Kcb can be 10% less than using Kc. The independent influence of soil evaporation and transpiration is important to better understand the water consumption of
young lime trees during growth compared to mature lime trees. 相似文献
993.
Bahiagrass (Paspalum notatum) is a warm-season grass used primarily in pastures and along highways and other low maintenance public areas in Florida.
It is also used in landscapes to some extent because of its drought tolerance. Bahiagrass can survive under a range of moisture
conditions from no irrigation to very wet conditions. Its well-watered consumptive use has not been reported previously. In
this study, bahiagrass crop coefficients (K
c) for an irrigated pasture were determined for July 2003 through December 2006 in central Florida. The eddy correlation method
was used to estimate crop evapotranspiration (ETc) rates. The standardized reference evapotranspiration (ETo) equation (ASCE-EWRI standardization of reference evapotranspiration task committee report, 2005) was applied to calculate
ETo values using on site weather data. Daily K
c values were estimated from the ratio of the measured ETc and the calculated ETo. The recommended K
c values for bahiagrass are 0.35 for January–February, 0.55 for March, 0.80 for April, 0.90 for May, 0.75 for June, 0.70 for
July–August, 0.75 for September, 0.70 for October, 0.60 for November, and 0.45 for December in central Florida. The highest
K
c value of 0.9 in May corresponded with maximum vapor pressure deficit conditions as well as cloud free conditions and the
highest incoming solar radiation as compared to the rest of the year. During the summer (June to August), frequent precipitation
events increased the cloud cover and reduced grass water use. The K
c annual trend was similar to estimated K
c values from another well-watered warm-season grass study in Florida. 相似文献
994.
Eddy covariance (EC) systems are being used to assess the accuracy of remote sensing methods in mapping surface sensible and
latent heat fluxes and evapotranspiration (ET) from local to regional scales, and in crop coefficient development. Therefore,
the objective was to evaluate the accuracy of EC systems in measuring sensible heat (H) and latent heat (LE) fluxes. For this
purpose, two EC systems were installed near large monolithic weighing lysimeters, on irrigated cotton fields in the Texas
High Plains, during the months of June and July 2008. Sensible and latent heat fluxes were underestimated with an average
error of about 30%. Most of the errors were from nocturnal measurements. Energy balance (EB) closure was 73.2–78.0% for daytime
fluxes. Thus, daylight fluxes were adjusted for lack of EB closure using the Bowen ratio/preservation of energy principle,
which improved the resulting EC heat flux agreement with lysimetric values. Further adjustments to EC-based ET included nighttime
ET (composite) incorporation, and the use of ‘heat flux source area’ (footprint) functions to compensate ET when the footprint
expanded beyond the crop field boundary. As a result, ET values remarkably matched lysimetric ET values, with a ‘mean bias
error ± root mean square error’ of −0.03 ± 0.5 mm day−1 (or −0.6 ± 10.2%). 相似文献
995.
A study was performed in order to evaluate the three-source model (Clumped model) for direct estimation of actual evapotranspiration
(ETa) and latent heat flux (LE) over a drip-irrigated Merlot vineyard trained on a vertical shoot positioned system (VSP) under
semi-arid conditions. The vineyard, with an average fractional cover of 30%, is located in the Talca Valley, Region del Maule,
Chile. The performance of the Clumped model was evaluated using an eddy covariance system during the 2006/2007 and 2007/2008
growing seasons. Results indicate that the Clumped model was able to predict ETa with a root mean square error (RMSE), mean bias error (MBE), and model efficiency (EF) of 0.33, −0.15 mm day−1 and 74%, respectively. Also, the Clumped model simulated the daytime variation of LE with a RMSE of 36 W m−2, MBE of −8 W m−2, and EF of 83%. Major disagreement (underestimated values) between observed and estimated values of ETa was found for clear days after rainfall or foggy days, but underestimated values were less than 10% of the data analysis.
The results obtained in this study indicate that the Clumped model could be used to directly estimate vine water requirements
for a drip-irrigated vineyard trained on a VSP. However, application of the Clumped model requires a good characterization
of the drip-irrigated vineyard architecture. 相似文献
996.
Hitoshi Fujiie Atsushi Maruyama Masako Fujiie Michiko Takagaki Douglas J. Merrey Masao Kikuchi 《Irrigation and Drainage Systems》2011,25(1):39-60
It is well-known that major irrigation projects have a strong scale economy, handicapping irrigation development in sub-Saharan
Africa (SSA) because of the difficulty in formulating large-scale projects. Using project-level investment cost and performance
data of major and minor irrigation projects, this paper examines the causes of the economy of scale phenomenon. We find that
strong scale economy exists not only for major but also for minor projects, i.e., small- and micro-scale, projects. This is
largely because of the existence of indivisible overhead costs such as high-opportunity-cost human resources for planning,
designing and engineering management and supervision. We also find that large differences between major and minor projects
in the absolute level of overhead as well as construction costs creates a strong scale diseconomy and results in better performance
of minor projects. The advantage of minor projects holds even when their higher risk associated with the water source is taken
into consideration. We argue that there is an urgent need to promote irrigation development in SSA through developing minor
projects, and to reduce the heavy burden of overhead costs by developing the capacity of human resources at the national,
local and farmer levels in the fields of irrigation engineering, irrigation agronomy, institutional development, and micro
water management technologies. 相似文献
997.
K. Palanisami Ruth Meinzen-Dick Mark Giordano Barbara Van Koppen C. R. Ranganathan 《Irrigation and Drainage Systems》2011,25(3):121-134
Irrigation tanks in India are common property resources. Tanks provide not only for irrigation, but also forestry, fishing,
domestic water supply, livestock, and other uses. Using empirical results from a study of tank performance from 80 tanks in
Tamil Nadu, South India in two time period: 1996-97 and 2009-10, this paper evaluates tank irrigation system performance in
terms of economic output and revenue generation forirrigation and other uses. The results indicate that irrigation and other
productive uses put together raised the total value of output at tank level by 12 % in 1996-97 and just 6 % in 2009-10. This
may suggest that tank multiple use values are small and getting smaller, and therefore not worth consideration. However, it
was also found that, while declining in absolute terms, non-irrigation uses provided the majority of tax revenues and still
more than cover government's operation and maintenance expenditure (O&M) budget. This finding provides another reason to consider
multiple use values and their linkage with overall system viability. 相似文献
998.
This paper introduces a combined modelling approach using a simple water budget model (THC-model) and a 3D reservoir sedimentation
model (MOHID Water) to adapt reservoir operation and visualise their effects on the sediment deposition. By this, an effective
combined sediment-water management can be identified under semi-arid conditions for dry, median and wet years. Results are
presented for the reservoirs of the Tuyamuyun Hydro-Complex (THC), which is located in the lower Amu Darya River. The determination
of the actual and usable reservoir storage volume shows that siltation will significantly adversely affect the ability of
the in-stream Channel Reservoir to regulate seasonal demand for both irrigation and municipal water supply. However, modelling
scenarios results confirm the effectiveness of adapted operation rules for the THC reservoirs and show that the operation
of large dams could be modified according to a combined sediment-water management. The experience gained during this study
emphasizes the fact that the concept of a combined reservoir management of sediments as well as water can be an efficient
measure to improve the sustainable long-term use of reservoirs and to contribute towards a safe water supply in water crisis
regions. 相似文献
999.
Optimizing yield,water requirements,and water productivity of aerobic rice for the North China Plain 总被引:1,自引:0,他引:1
Changying Xue Xiaoguang Yang Bas A. M. Bouman Wei Deng Qiuping Zhang Weixiong Yan Tianyi Zhang Aji Rouzi Huaqi Wang 《Irrigation Science》2008,26(6):459-474
Water resources for agriculture are rapidly declining in the North China Plain because of increasing industrial and domestic
use and because of decreasing rainfall resulting from climate change. Water-efficient agricultural technologies need to be
developed. Aerobic rice is a new crop production system in which rice is grown in nonflooded and nonsaturated aerobic soil,
just like wheat and maize. Although an estimated 80,000 ha are cultivated with aerobic rice in the plain, there is little
knowledge on obtainable yields and water requirements to assist farmers in improving their management. We present results
from field experiments with aerobic rice variety HD297 near Beijing, from 2002 to 2004. The crop growth simulation model ORYZA2000
was used to extrapolate the experimental results to different weather conditions, irrigation management, and soil types. We
quantified yields, water inputs, water use, and water productivities. On typical freely draining soils of the North China
Plain, aerobic rice yields can reach 6–6.8 t ha−1, with a total water input ranging between 589 and 797 (rainfall = 477 m and water application = 112–320 mm). For efficient
water use, the irrigation water can be supplied in 2–4 applications and should aim at keeping the soil water tension in the
rootzone below 100–200 kPa. Under those conditions, the amount of water use by evapotranspiration was 458–483 mm. The water
productivity with respect to total water input (irrigation plus rainfall) was 0.89–1.05 g grain kg−1 water, and with respect to evapotranspiration, 1.28–1.42 g grain kg−1 water. Drought around flowering should be avoided to minimize the risk of spikelet sterility and low grain yields. The simulations
suggest that, theoretically, yields can go up to 7.5 t ha−1 and beyond. Further research is needed to determine whether the panicle (sink) size is large enough to support such yields
and/or whether improved management is needed. 相似文献
1000.