Monthly data records of 40 Iranian stations distributed over the country, for the period 1971-2005, were utilized for estimation of reference evapotranspiration (ET o ) using Penman-Monteith (PM-ET o ), Hargreaves-Samani (HS) and FAO-PM temperature (PMT) methods. To estimate ET o with HS and PMT methods, appropriate k Rs , an empirical radiation adjustment coefficient, were considered for each station, whereas T min was adjusted for estimation of T dew and used only for PMT computation. It was found that the appropriate k Rs for both HS and PMT methods are identical for a given station and it is generally smaller in sub-humid and humid than in semi-arid to hyper-arid climates. The performance of the PMT was further improved in both arid and humid climates when T min was adjusted. The result suggested that the HS and PMT methods appropriately predict ET o for all climatic regions of Iran if the appropriate k Rs was utilized. However, the considered methods showed weak performances for some stations in arid and hyper-arid climates of eastern and southern Iran owing to the effect of extreme and variable wind speed inherent in the PM-ET o . Thus, the role played by wind speed in ET o estimation was examined; the result indicated that the existence of extreme winds, and also the time variability of wind speed, is responsible for the observed discrepancies between PMT and PM-ET o estimates. The spatial patterns of ET o computed with HS and PMT methods found to be identical and resemble to that of PM-ET o , all showing a gradual increasing from north to south, with the lowest ET o values observed over northern humid and sub-humid climates of Iran and larger ET o for arid and hyper-arid climates in the southern and eastern country. Results indicated that the HS and PMT methods are appropriate alternatives for estimation of ET o for all climatic regions of Iran.

An analysis of drought in western Iran from 1966 to 2000 is presented using monthly precipitation data observed at 140 gauges uniformly distributed over the area. Drought conditions have been assessed by means of the Standardized Precipitation Index (SPI). To study the long-term drought variability the principal component analysis was applied to the SPI field computed on 12-month time scale. The analysis shows that applying an orthogonal rotation to the first two principal component patterns, two distinct sub-regions having different climatic variability may be identified. Results have been compared to those obtained for the largescale using re-analysis data suggesting a satisfactory agreement. Furthermore, the extension of the large-scale analysis to a longer period shows that the spatial patterns and the associated time variability of drought are subjected to noticeable changes. Finally, the relationship between hydrological droughts in the two sub-regions and El Niño Southern Oscillation events has been investigated finding that there is not clear evidence for a link between the two phenomena.

ABSTRACT:The relationships between large-scale atmospheric circulation types and seasonal regimes of daily precipitation over Iran are assessed using daily precipitation from a high-resolution gridded dataset provided by the Asian Precipitation-Highly Resolved Observational Data Integration Towards the Evaluation of Water Resources (APHRODITE) Project. Regional spatial modes of daily precipitation variability were identified by S-mode Principal Component Analysis (PCA) with Varimax rotation, applied to the subset of days when at least 10% of all grid-points over Iran received precipitation ≥5 mm. The study refers to the period 1961-2004 and is carried out for each season (excluding summer) separately. To characterize the dynamical features associated with each regional precipitation regime (PR), composites of daily atmospheric fields are computed by only averaging days with rotated PCA scores ≥1.5 (strong positive phase). In autumn and winter, Iran is divided into five PRs, while four PRs are identified in spring. Results suggest that the spatial distribution of precipitation over Iran is largely governed by the geographical position of both the mid-tropospheric trough over the Middle East and the Arabian anticyclone. In fact, in almost all PRs, the trough, as a pre-conditioning factor, leads to regional-scale ascending motions, whereas the Arabian anticyclone induces low-tropospheric moisture transports from southern water bodies into the cyclonic systems near Iran, triggering rain-generating conditions.

Abstract. The spatial variability of precipitation and drought are investigated for Portugal using monthly precipitation from 74 stations and minimum and maximum temperature from 27 stations, covering the common period of . Seasonal precipitation and the corresponding percentages in the year, as well as the precipitation concentration index (PCI), was computed for all 74 stations and then used as an input matrix for an R-mode principal component analysis to identify the precipitation patterns. The standardized precipitation index at 3 and 12 month time scales were computed for all stations, whereas the Palmer Drought Severity Index (PDSI) and the modified PDSI for Mediterranean conditions (MedPDSI) were computed for the stations with temperature data. The spatial patterns of drought over Portugal were identified by applying the S-mode principal component analysis coupled with varimax rotation to the drought indices matrices. The result revealed two distinct sub-regions in the country relative to both precipitation regimes and drought variability. The analysis of time variability of the PC scores of all drought indices allowed verifying that there is no linear trend indicating drought aggravation or decrease. In addition, the analysis shows that results for SPI-3, SPI-12, PDSI and MedPDSI are coherent among them.