Browsing by Author "Pereira, L.S."
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- Are drought occurrence and severity aggravating ? A study on SPI drought class transitions using log-linear models and ANOVA-like inferencePublication . Moreira, E.E.; Mexia, J.T.; Pereira, L.S.Abstract. Long time series (95 to 135 yr) of the 12-month time scale Standardized Precipitation Index (SPI) relative to 10 locations across Portugal were studied with the aim of investigating if drought frequency and severity are changing through time. Considering four drought severity classes, time series of drought class transitions were computed and later divided into several sub-periods according to the length of SPI time series. Drought class transitions were calculated to form a 2-dimensional contingency table for each sub-period, which refer to the number of transitions among drought severity classes. Two-dimensional log-linear models were fitted to these contingency tables and an ANOVA-like inference was then performed in order to investigate differences relative to drought class transitions among those subperiods, which were considered as treatments of only one factor. The application of ANOVA-like inference to these data allowed to compare the sub-periods in terms of probabilities of transition between drought classes, which were used to detect a possible trend in droughts frequency and severity. Results for a number of locations show some similarity between alternate sub-periods and differences between consecutive ones regarding the persistency of severe/extreme and sometimes moderate droughts. In global terms, results do not support the assumption of a trend for progressive aggravation of drought occurrence during the last century, but rather suggest the existence of long duration cycles.
- Assessing and modelling water use and the partition of evapotranspiration of irrigated hop (Humulus lupulus) and relations of transpiration with hops yield and alpha-acidsPublication . Fandiño, M.; Olmedo, J.L.; Martinez, E.M.; Valladares, J.; Paredes, Paula; Rey, B.J.; Mota, M.; Cancela, J.J.; Pereira, L.S.This study was conducted during three seasons (2012–2014) in an experimental hop yard at Mabegondo,Galicia, NW Spain. The research aimed at calibrating and validating the soil water balance model SIMD-ualKc for Humulus lupulus L. cv. ‘Nugget’. The model computes the soil water balance using the dualKcapproach, thus partitioning crop evapotranspiration (ETc) into crop transpiration, ground covertranspiration and soil evaporation. Calibration and validation were performed using TDR soil watercontent measurements, which produced small root mean square errors (RMSE) ranging from 0.012 to0.015 cm3cm−3. The initial, mid-season and end-season basal crop coefficients (Kcb) that allow com-puting hop transpiration were respectively 0.16, 0.97 and 0.83. The single Kcfor the same crop growthstages, which refers to transpiration and soil evaporation together, were respectively 0.69, 1.02 and 0.85.SIMDualKc provided to estimate water use by the hop yard and the components of the soil water bal-ance, particularly hop transpiration (THop), ground covered transpiration (Tcover) and soil evaporation (Es).THoprepresented 92% of actual evapotranspiration (ETc act) during the mid-season, and Esaveraged 69% ofETc actduring the initial stage. It was observed that Tcoverwas strongly influenced by soil and ground covermanagement. The impacts of water use and THopon hop yield quantity and quality were assessed. A lin-ear regression between hop cone yield and THophas been found, with a high coefficient of determinationr2= 0.92, while the linear regressions of THopwith alpha and beta-acids had regression coefficients notsignificantly different from zero. These results denote appropriate irrigation management with absenceof stresses that could affect yields or the concentration of bitter acids
- Assessing economic impacts of deficit irrigation as related to water productivity and water costsPublication . Pereira, L.S.; Rodrigues, G.S.This study aims at assessing the feasibility of deficit irrigation of maize, wheat and sunflower through an analysis of the economic water productivity (EWP). It focuses on selected sprinkler-irrigated fields in Vigia Irrigation District, Southern Portugal. Various scenarios of water deficits and water availability were considered. Simulations were performed for average, high and very high climatic demand. The potential crop yields were estimated from regional climatic data and local information. Using field collected data on yield values, production costs, water costs, commodity prices and irrigation performance, indicators on EWP were calculated. Results show that a main bottleneck for adopting deficit irrigation is the presently low performance of the irrigation systems used in the considered fields, which leads to high water use and low EWP. Decreasing water use through deficit irrigation also decreases the EWP. Limited water deficits for maize are likely to be viable when the irrigation performance is improved if water prices do not increase much, and the commodity price does not return to former low levels. The sunflower crop, despite lower sensitivity to water deficits than maize, does not appear to be a viable solution to replace maize when water restrictions are high; however it becomes an attractive crop if recently high commodity prices are maintained. With improved irrigation performance, wheat deficit irrigation is viable including when full water costs are applied, if former low prices are not returned to. However, under drought conditions full water costs are excessive. Thus, adopting deficit irrigation requires not only an appropriate irrigation scheduling but higher irrigation performance, and that the application of a water prices policy would be flexible, thus favouring the improvement of the irrigation systems.
- Assessing economic impacts of deficit irrigation as related to water productivity and water costsPublication . Rodrigues, Gonçalo C.; Pereira, L.S.This study aims at assessing the feasibility of deficit irrigation of maize, wheat and sunflower through an analysis of the economic water productivity (EWP). It focuses on selected sprinkler-irrigated fields in Vigia Irrigation District, Southern Portugal. Various scenarios of water deficits and water availability were considered. Simulations were performed for average, high and very high climatic demand. The potential crop yields were estimated from regional climatic data and local information. Using field collected data on yield values, production costs, water costs, commodity prices and irrigation performance, indicators on EWP were calculated. Results show that a main bottleneck for adopting deficit irrigation is the presently low performance of the irrigation systems used in the considered fields, which leads to high water use and low EWP. Decreasing water use through deficit irrigation also decreases the EWP. Limited water deficits for maize are likely to be viable when the irrigation performance is improved if water prices do not increase much, and the commodity price does not return to former low levels. The sunflower crop, despite lower sensitivity to water deficits than maize, does not appear to be a viable solution to replace maize when water restrictions are high; however it becomes an attractive crop if recently high commodity prices are maintained. With improved irrigation performance, wheat deficit irrigation is viable including when full water costs are applied, if former low prices are not returned to. However, under drought conditions full water costs are excessive. Thus, adopting deficit irrigation requires not only an appropriate irrigation scheduling but higher irrigation performance, and that the application of a water prices policy would be flexible, thus favouring the improvement of the irrigation systems
- Assessing spatial variability and trends of droughts in Eastern Algeria using SPI, RDI, PDSI, and MedPDSI—A novel drought index using the FAO56 evapotranspiration methodPublication . Merabti, Abdelaaziz; Darouich, Hanaa; Paredes, Paula; Meddi, Mohamed; Pereira, L.S.Drought is one of the most severe natural disasters worldwide, but with a particular emphasis in sub-humid and semi-arid climates. Several indices have been created to appropriately identify drought’s characteristics and variability. The main objectives of this study consisted of analyzing the behavior of different indices applied in northeast Algeria and comparing them across a long-term data set (1961–2014). The SPI and RDI at 9-month time scales were compared to the PDSI and MedPDSI based on 123 rainfall stations and gridded PET data interpolated to all the locations. A principal component analysis (PCA) in S-mode with varimax rotation (RPC) was applied to the monthly values of all indices to analyze the spatiotemporal patterns of droughts. Two principal components were retained, which identified two sub-regions with coherent differences related to their distance from the Mediterranean Sea and the UNEP aridity index. Trends in the RPC scores were assessed using the modified Mann–Kendall (MMK) test and Sen’s slope estimator, which showed a fundamental difference between the two sub-regions. The RPC of all drought indices showed trends of decreases in the frequency and severity of droughts in the northern sub-region, and trends of increases in the frequency and severity of droughts in the southern region, where the climate is mostly semi-arid and arid. Only a few cases were statistically significant, mostly when using the PDSI and MedPDSI for the southern sub-region. The spatial patterns of moderate, severe, and extreme drought occurrences were similar for the SPI and RDI pair of indices based on the probability of rainfall anomalies, and for the Sc-PDSI and MedPDSI pair based on water balance anomalies. The interpretation of the spatial variability of droughts, mainly of the extreme ones, was supported by an analysis of semi-variograms. The novel index MedPDSI compared well with the other indices and showed advantages of performing the soil water balance following the FAO56 dual Kc method with the actual olive evapotranspiration instead of PET, and of better explaining the spatial variability of extreme droughts; in addition, the trends detected were significant for both the northern and southern sub-regions.
- Assessing the performance of the FAO Aquacrop model to estimate maize yields and water use under full and deficit irrigation with focus on model parametrizationPublication . Paredes, Paula; Melo-Abreu, J.P.; Alves, I.; Pereira, L.S.tSeveral maize field experiments, including deficit and full irrigation, were performed in Ribatejo region,Portugal and were used to assess water stress impacts on yields using the AquaCrop model. The modelwas assessed after its parameterization using field observations relative to leaf area index (LAI), cropevapotranspiration, soil water content, biomass and final yield data and also using default parameters. LAIdata were used to calibrate the canopy cover (CC) curve. Results showed that when the CC curve is properlycalibrated, with root mean square errors (RMSE) smaller than 7.4%, model simulations, namely relative tocrop evapotranspiration and its partition, show an improved accuracy. The model performance relative tosoil water balance simulation revealed a bias in estimation but low estimation errors, with RMSE < 13% ofthe total available soil water. However the model tends to overestimate transpiration and underestimatesoil evaporation. A good model performance was obtained relative to biomass and yield predictions, withRMSE lower than 11% and 9% of the average observed biomass and yield, respectively. Overall results showadequacy of AquaCrop for estimating maize biomass and yield under deficit irrigation conditions, mainlywhen an appropriate parameterization is adopted. The model showed less good performance when usingthe default parameters but errors are likely acceptable when field data are not available
- Basin irrigation design with multi-criteria analysis focusing on water saving and economic returns: application to wheat in Hetao, Yellow River basinPublication . Miao, Quingfeng; Shi, Haibin; Gonçalves, José M.; Pereira, L.S.The sustainability of the Hetao Irrigation System, located in the water scarce upper Yellow River basin, is a priority considering the need for water saving, increased water productivity, and higher farmers’ incomes. The upgrading of basin irrigation, the main irrigation method, is essential and includes the adoption of precise land levelling, cut-off management, improved water distribution uniformity, and adequate irrigation scheduling. With this objective, the current study focuses on upgrading wheat basin irrigation through improved design using a decision support system (DSS) model, which considers land parcels characteristics, crop irrigation scheduling, soil infiltration, hydraulic simulation, and environmental and economic impacts. Its use includes outlining water saving scenarios and ranking alternative designs through multi-criteria analysis considering the priorities of stakeholders. The best alternatives concern flat level basins with a 100 and 200 m length and inflow rates between 2 and 4 L s1 m1. The total irrigation cost of designed projects, including the cost of the autumn irrigation, varies between 2400 and 3300 Yuan ha1; the major cost component is land levelling, corresponding to 33–46% of total irrigation costs. The economic land productivity is about 18,000 Yuan ha1. The DSS modelling defined guidelines to be applied by an extension service aimed at implementing better performing irrigation practices, and encouraged a good interaction between farmers and theWater Users Association, thus making easier the implementation of appropriate irrigation management programs
- Climate trends and behaviour of drought indices based on precipitation and evapotranspiration in PortugalPublication . Paulo, A.A.; Rosa, R.D.; Pereira, L.S.Distinction between drought and aridity is crucial to understand water scarcity processes. Drought indices are used for drought identification and drought severity characterisation. The Standardised Precipitation Index (SPI) and the Palmer Drought Severity Index (PDSI) are the most known drought indices. In this study, they are compared with the modified PDSI for Mediterranean conditions (MedPDSI) and the Standardised Precipitation Evapotranspiration Index (SPEI). MedPDSI results from the soil water balance of an olive crop, thus real evapotranspiration is considered, while SPEI uses potential (climatic) evapotranspiration. Similarly to the SPI, SPEI can be computed at various time scales. Aiming at understanding possible impacts of climate change, prior to compare the drought indices, a trend analysis relative to precipitation and temperature in 27 weather stations of Portugal was performed for the period 1941 to 2006. A trend for temperature increase was observed for some weather stations and trends for decreasing precipitation in March and increasing in October were also observed for some locations. Comparisons of the SPI and SPEI at 9- and 12-month time scales, the PDSI and Med- PDSI were performed for the same stations and period. SPI and SPEI produce similar results for the same time scales concerning drought occurrence and severity. PDSI and Med- PDSI correlate well between them and the same happened for SPI and SPEI. PDSI and MedPDSI identify more severe droughts than SPI or SPEI and identify drought occurrence earlier than these indices. This behaviour is likely to be related with the fact that a water balance is performed with PDSI and MedPDSI, which better approaches the supplydemand balance.
- Comparing sprinkler and drip irrigation systems for full and deficit irrigated maize using multicriteria analysis and simulation modelling: ranking for water savings vs. farm economic returnsPublication . Rodrigues, Gonçalo C.; Paredes, Paula; Gonçalves, José M.; Alves, Isabel; Pereira, L.S.This study aims to assess the economic feasibility of full and deficit irrigated maize using center pivot, set sprinkler systems and drip tape systems through multicriteria analysis. Different irrigation treatments were evaluated and compared in terms of beneficial water use and physical and economical water productivity for two commodity prices and three irrigation systems scenarios applied to a medium and a large field of 5 and 32 ha respectively. Results show that deficit treatments may lead to better water productivity indicators but deficit irrigation (DI) feasibility is highly dependent on the commodity prices. Various well-designed and managed pressurized irrigation systems’ scenarios – center-pivot, set sprinkler systems and drip tape systems – were compared and ranked using multicriteria analysis. For this, three different prioritization schemes were considered, one referring to water savings, another relative to economic results, and a third one representing a balanced situation between the first two. The rankings of alternative solutions were very sensitive to the decision-maker priorities, mainly when comparing water saving and economic results because the selected alternatives were generally not common to both priority schemes. However, some of the best alternatives for the balanced priorities scheme are common to the other two, thus suggesting a possible trade-off when selecting the best alternatives. Deficit irrigation strategies also rank differently for the various scenarios considered. The study shows that deficit irrigation with exception of mild DI is generally not economically feasible. The adoption of well designed and managed irrigation systems requires consideration of priorities of farm management in terms of water saving and economic results since that some water saving solutions do not allow appropriate recover of the investment costs, particularly with DI. Basing decisions upon multicriteria analysis allows farmers and decision-makers to better select irrigation systems and related management decisions. Results also indicate that appropriate support must be given to farmers when adopting high performance but expensive irrigation systems aimed at sustainable crop profitability
- Comparing sprinkler and surface irrigation for wheat using multi-criteria analysis: water saving vs. economic returnsPublication . Darouich, Hanaa; Cameira, Maria do Rosário; Gonçalves, José M.; Paredes, Paula; Pereira, L.S.Coping with water scarcity using supplemental irrigation of wheat (Triticum aestivum L.) in the semi-arid northeast Syria is a great challenge for sustainable water use in agriculture. Graded borders and set sprinkler systems were compared using multi-criteria analysis. Alternative solutions for surface irrigation and for sprinkler systems were developed with the SADREG and the PROASPER design models, respectively. For each alternative, two deficit irrigation strategies were considered, which were characterized using indicators relative to irrigation water use, yields and water productivity, including farm economic returns. Alternatives were ranked considering two contrasting priorities: economic returns and water saving. A first step in ranking led to a selection of graded borders with and without precise land levelling and of solid set and semi-permanent sprinkler systems. Precise-levelled borders were better for water saving, while non-precise ones ranked higher for economic returns. Semi-permanent set systems have been shown to be better in economic terms and similar to solid set systems when water saving is prioritized. Semi-permanent sprinkler systems rank first when comparing all type of systems together regardless of the considered deficit irrigation strategy. Likely, border irrigation is appropriate when wheat is in rotation with cotton if the latter is surface irrigated. When peace becomes effective, appropriate economic incentives and training for farmers are required to implement innovative approaches