A carregar...
Projeto de investigação
Not Available
Financiador
Autores
Publicações
Daily grass reference evapotranspiration with Meteosat Second Generation shortwave radiation and reference ET products
Publication . Paredes, Paula; Trigo, Isabel; Bruin, Henk de; Simões, Nuno; Pereira, L.S.
This study assesses the accuracy of estimating daily grass reference evapotranspiration (PM-ETo) using daily
shortwave radiation (Rs) and reference evapotranspiration (ETREF) products provided by the Meteosat Second
Generation (MSG) geostationary satellite delivered by the Satellite Applications Facility on Land Surface Analysis
(LSA-SAF) framework. The accuracy of using reanalysis ERA5 shortwave radiation data (Rs ERA5) provided by the
European Center for Medium-Range Weather Forecasts (ECMWF) is also evaluated. The assessments were performed
using observed weather variables at 37 weather stations distributed across continental Portugal, where
climate conditions range from semi-arid to humid, and 12 weather stations located in Azores islands, characterized
by humid, windy and often cloudy conditions. This study’s use of data from a variety of climate
conditions contributed to a unique and innovative assessment of the usability of LSA-SAF and ERA5 products for
ETo estimation. The first assessment focused on comparing LSA-SAF estimates of Rs (Rs LSA-SAF) against ground
stations (Rs ground). The results showed a good matching between the two Rs data sets for continental Portugal but
a tendency for Rs LSA-SAF to under-estimate Rs ground in the cloudy islands of Azores. ETo values computed using
Rs LSA-SAF data and observed temperature, humidity and wind speed (ETo LSA-SAF) were then compared with PMETo
estimates with ground-based data, which were used as benchmark; input data of temperature and humidity
needed for PM-ETo were quality checked for surface aridity effects. It was observed that ETo LSA-SAF is strongly
correlated with PM-ETo (R2 > 0.97) for most locations in continental Portugal, with regression coefficient of a
linear regression forced to the origin ranging between 0.95 and 1.05, mean root mean square error (RMSE)
of 0.13 mm d 1, and Nash and Sutcliff efficiency of modeling (EF) above 0.95. For most Azores locations,
ETo LSA-SAF over-estimated PM-ETo. This is likely a consequence of the high spatio-temporal heterogeneity of
weather conditions that occur in these oceanic islands together with the different footprints of satellite (averaged
over the pixel) and station observations. Reanalysis ERA5 shortwave radiation data presented similar behavior to
the LSA-SAF products, however with slightly lower accuracy. The daily LSA-SAF ETREF product (ETREF LSA-SAF)
was assessed and results have shown a good accuracy of this product, with acceptable RMSE and high EF values,
for continental Portugal but a low accuracy for the Azores islands. A simplified bias correction approach
was shown to improve both ETo derived from the LSA-SAF products, namely for Azores stations, which seem
to be representative of smaller areas. The use of the FAO-PM temperature approach (PMT) was also assessed
using the Rs LSA-SAF and Rs ERA5 data, which showed a superiority of the LSA-SAF product for ETo estimations
(ETo PMT LSA-SAF). No significant differences (p < 0.05) were observed in terms of the median value of the RMSE
when adopting ETo PMT and ETREF LSA-SAF. Differently, results showed that using the Rs LSA-SAF in the PMT
approach (ETo PMT LSA-SAF) produces significantly better RMSE results than ETo PMT and ETREF LSA-SAF. Overall, the
performed assessment allows concluding that the use of Rs LSA-SAF, and to a lesser extent the use of the
Rs ERA5, highly improves the accuracy of computation of ETo when Rs observations are not available, including
when only temperature data are accessible. The use of the ETREF LSA-SAF product is a good alternative when observed weather data are not available
Actual and standard crop coefficients for semi‑natural and planted grasslands and grasses: a review aimed at supporting water management to improve production and ecosystem services
Publication . Pereira, Luís Santos; Paredes, Paula; Espírito-Santo, Dalila; Salman, Maher
Natural and planted grasslands play a very important role in agriculture as source of various ecosystem services, including
carbon sequestration and biodiversity, and are responsible for a large fraction of agricultural water use in rainfed and irrigated
fields. It is, therefore, relevant to precisely know their water use and vegetation requirements with consideration of relevant
climate, from extremely cold, dry, with long winter seasons, to tropical humid and hot climates, thus with a large variability
of vegetation. Semi-natural grasslands are basically used for grazing and mainly refer to highland pastures and meadows,
steppes, savannas, pampas, and mixed forest systems. The FAO method to compute crop (vegetation) evapotranspiration (ETc)
through the product of a crop coefficient (K c ) by the reference evapotranspiration (ETo ) is adopted. The selected papers were
those where actual ETc (ETc act ) was derived from field observations and ETo was computed with the FAO56 definition, or
with another method that could be referred to the former. Field derived ETc act methods included soil water balance, Bowen
ratio and eddy covariance measurements, as well as remote sensing vegetation indices or surface energy balance models,
thus reviewed Kc act (ETc act/ETo) values were obtained from field data. These Kc act refer to initial, mid-season and end season
(K c act ini , K c act mid , K c act end ) when reported values were daily or monthly; otherwise, only average values (K c act avg ) were
collected. For cases relative to cold or freezing winters, data refer to the warm season only. For grasses cut for hay, K c act ini ,
Kc act mid , and Kc act end refer to a cut cycle. Kc act values rarely exceeded 1.25, thus indicating that field measurements reported
did respect the available energy for evaporation. Overall, K c act mid for semi-natural grasslands in cold climates were lower
than those in hot climates except when available water was high, with K c act mid for meadows and mountain pastures gener-
ally high. Steppes have K c act mid values lower than savannas. Grasses commonly planted for hay and for landscape generally
showed high K c act mid values, while a larger variability was observed with grasses for grazing. The collected K c act values
were used to define standard Kc values for all grassland and grasses. Nevertheless, the tabulated Kc act are indicative values of
K c to be used for actual water management purposes and/or irrigation scheduling of planted grasslands. It is expected that a
better knowledge of the standard and/or indicative K c values for a wide variety of grasslands and grasses will support better
management aimed to improve grass productivity and ecosystem services, including biodiversity and carbon sequestration
Is irrigation water an overlooked source of nitrogen in agriculture?
Publication . Serra, J.; Paredes, P.; Cordovil, C. M. d S.; Cruz, S.; Hutchings, N. J.; Cameira, M. R.
The increase of agricultural nitrogen (N) inputs since the 1960s is a key driver in surface- and groundwater
nitrate pollution. The water abstracted from these sources can input substantial amounts of reactive nitrogen
(NIrrig) if used for crop irrigation. This input is often not included in N related agricultural policies and studies,
which are likely underestimating the magnitude of N pollution hotspots and overestimating the N use efficiency.
In this study, we provided prima facie evidence that NIrrig is a neglected source of N in irrigated systems. The NIrrig
was computed for 278 municipalities in mainland Portugal along the period 1995–2019 based on the gross
irrigation requirements and nitrate concentration in ground- and surface water sources. The former was derived
using two complementary approaches, using the AquaCrop and GlobWat models, while the latter were computed
following spatially explicit approaches. NIrrig showed annual large fluctuations (6–11 Gg N yr-1), of which 91%
was from groundwater sources. Results show that NIrrig averaged 14 ( ± 11) kg N ha-1 yr-1, which is equivalent to
3 ( ± 4) % of the N in synthetic fertilisers. This input was higher in the municipalities that simultaneously present
high irrigation demand and the nitrate-contaminated groundwater as an irrigation source. In these cases, located
in Nitrate Vulnerable Zones, NIrrig reached up to 95 kg N ha-1 yr-1 and more than 80% of the N in synthetic
fertilizers. This study highlights the importance of linking water and nutrient policies to better gain insight on
NIrrig, for which the current study provided for a simple modelling framework.
A review of strategies, methods and technologies to reduce non-beneficial consumptive water use on farms considering the FAO56 methods
Publication . Jovanovic, N.; Pereira, L.S.; Paredes, Paula; Pôças, I.; Cantore, V.; Todorovic, M.
In the past few decades, research has developed a multitude of strategies, methods and technologies to reduce
consumptive water use on farms for adaptation to the increasing incidence of water scarcity, agricultural
droughts and multi-sectoral competition for water. The adoption of these water-saving practices implies accurate
quantification of crop water requirements with the FAO56 crop coefficient approach, under diverse water
availability and management practices. This paper critically reviews notions and means for maintaining high
levels of water consumed through transpiration, land and water productivity, and for minimizing non-beneficial
water consumption at farm level. Literature published on sound and quantified experimentation was used to
evaluate water-saving practices related to irrigation methods, irrigation management and scheduling, crop
management, remote sensing, plant conditioners, mulching, soil management and micro-climate regulation.
Summary tables were developed on the benefits of these practices, their effects on non-beneficial water consumption,
crop yields and crop water productivity, and the directions for adjustment of FAO56 crop coefficients
when they are adopted. The main message is that on-farm application of these practices can result in water
savings to a limited extent (usually<20%) compared to sound conventional practices, however this may
translate into large volumes of water at catchment scale. The need to streamline data collection internationally
was identified due to the insufficient number of sound field experiments and modelling work on the FAO56 crop
water requirements that would allow an improved use of crop coefficients for different field conditions and
practices. Optimization is required for the application of some practices that involve a large number of possible
combinations (e.g. wetted area in micro-irrigation, row spacing and orientation, plant density, different types of
mulching, in-field water harvesting) and for strategies such as deficit irrigation that aim at balancing water
productivity, the economics of production, infrastructural and irrigation system requirements. Further research
is required on promising technologies such as plant and soil conditioners, and remote sensing applications
Transpiration and Water Use of an Irrigated Traditional Olive Grove with Sap-Flow Observations and the FAO56 Dual Crop Coefficient Approach
Publication . Puig-Sirera, Angela; Rallo, Giovanni; Paredes, Paula; Paço, Teresa; Minacapilli, Mario; Provenzano, Giuseppe; Pereira, L.S.
The SIMDualKc model was applied to evaluate the crop water use and the crop coefficient
(Kc) of an irrigated olive grove (Olea europaea L.) located in Sicily, Italy, using experimental data
collected from two crop seasons. The model applies the FAO56 dual Kc approach to compute the
actual crop evapotranspiration (ETc act) and its components, i.e., the actual tree transpiration (Tc act),
obtained through the basal crop coefficient (Kcb), and soil evaporation according to an evaporation coefficient
(Ke). Model calibration was performed by minimizing the difference between the predicted
Tc act and the observed daily tree transpiration measured with sap flow instrumentation (TSF field)
acquired in 2009. The validation was performed using the independent data set of sap flow measurements
from 2011. The calibrated Kcb was equal to 0.30 for the initial and non-growing season stages,
0.42 for the mid-season, and 0.37 for the end season. For both seasons, the goodness-of-fit indicators
relative to comparing TSF field with the simulated Tc act resulted in root mean square errors (RMSE)
lower than 0.27 mm d1 and a slope of the linear regression close to 1.0 (0.94 b0 1.00). The olive
grove water balance simulated with SIMDualKc produced a ratio between soil evaporation (Es) and
ETc act that averaged 39%. The ratio between actual (ETc act) and potential crop evapotranspiration
(ETc) varied from 84% to about 99% in the mid-season, indicating that the values of ETc act are close
to ETc, i.e., the adopted deficit irrigation led to limited water stress. The results confirm the suitability
of the SIMDualKc model to apply the FAO56 dual Kc approach to tree crops, thus assessing the water
use of olives and supporting the development of appropriate irrigation management tools that are usable
by farmers. A different way to estimate Kcb is based on the approach suggested in 2009 by Allen
and Pereira (A&P), which involves the measured fraction of ground covered (shaded) by the crop
and the height of the trees. Its application to the studied grove produced the mid-season Kcb values
ranging from 0.40–0.45 and end-season Kcb values ranging from 0.35–0.40. The comparison between
the A&P-computed Tc act A&P and TSF field shows RMSE values ranging from 0.27 to 0.43 mm d1,
which demonstrates the adequacy of the latter approach for parameterizing water balance models
and for irrigation scheduling decision making
Unidades organizacionais
Descrição
Palavras-chave
Contribuidores
Financiadores
Entidade financiadora
Fundação para a Ciência e a Tecnologia
Programa de financiamento
DL 57/2016
Número da atribuição
DL 57/2016/CP1382/CT0022