As the need for climatic data is increasing in times of climate change and water scarcity, remote sensing (RS) and hydrological modelling are ways to battle these problems, especially in data scarce areas. The actual evapotranspiration (ETa) is one of the key parameters when assessing the water balance and a good estimate of this parameter is thus of great importance. In this study a hydrologicalmodel was created with the Soil and Water Assessment Tool (SWAT) over the Malwathu Oya river basin, Sri Lanka, and the SWAT ETa estimates were compared to RS derived ETa from FAO’s open access database WaPOR. A sensitivity analysis and a calibration with observed streamflow data of the SWAT model was conducted with the SUFI-2 algorithm in SWAT-CUP. The calibration was satisfactory and showed the following values for the performance parameters: R2 = 0.72, Nash-Sutcliffe Efficiency, NSE = 0.69, and Percent of Bias, PBIAS = -10.4. The most sensitive parameters were CN2 (runoff curve number for moisture condition II), SOL_AWC (soil available water capacity), and ESCO (soil evaporation compensation factor). The water balance partitioning from the calibrated SWAT model showed a ratio of 0.68 between ETa and precipitation as an annual average between 2012–2020. In the comparison between SWAT ETa and WaPOR ETa the SWAT ETa showed a clear underestimation, particularly during the drier Yala growing season (May – August). However, the SWAT land use classes representing the cultivated rice fields agreed well with WaPOR while the forest and range grasses were underpredicted. To increase the performance of SWAT in estimating ETa the following was recommended: improvement of the simulation of the shallow aquifers, more accurate forest parameters, deactivation of the default dormancy period in SWAT, calibration with ETa instead of streamflow, and a higher resolution soil map together with more soil measurements. / Eftersom behovet av klimatdata ökar i tider av klimatförändringar och vattenbrist är fjärranalys (RS) och hydrologisk modellering exempel på metoder för att lösa dessa problem, särskilt i områden med brist på data. Den faktiska evapotranspirationen (ETa) är en nyckelparameter vid bedömning av vattenbalansen och en bra uppskattning av denna parameter är därför av stor betydelse. I denna studie skapades en hydrologisk modell med Soil and Water Assessment Tool (SWAT) över avrinningsområdet Malwathu Oya i Sri Lanka, och SWAT ETa -uppskattningarna jämfördes med RS-beräknad ETa från FAO:s öppna databas WaPOR. En känslighetsanalys och en kalibrering med observerade flödesdata av SWAT-modellen utfördes med SUFI-2-algoritmen i SWAT-CUP. Kalibreringen var tillfredsställande och visade följande värden för prestandaparametrarna: R2 = 0,72, Nash-Sutcliffe-Efficiency, NSE = 0,69 och Percent of Bias, PBIAS = -10,4. De mest känsliga parametrarna var CN2 (avrinningskurvtal för fukttillstånd II), SOL_AWC (jordens tillgängliga vattenkapacitet) och ESCO (kompensationsfaktor för markavdunstning). Vattenbalansfördelningen från den kalibrerade SWAT-modellen visade ett förhållande på 0,68 mellan ETa och nederbörden som ett årligt medelvärde mellan 2012–2020. I jämförelsen mellan SWAT ETa och WaPOR ETa visade SWAT ETa en tydlig underskattning, särskilt under den torrare Yala-växtsäsongen (maj – augusti). Däremot överensstämde SWAT-markanvändningsklasserna som representerade de odlade risfälten väl med WaPOR medan skog och gräsfälten var underskattade. För att öka prestandan för SWAT vid uppskattning av ETa rekommenderades följande: förbättring av simuleringen av de grunda akvifärerna, förbättrade skogsparametrar, inaktivering av den automatiska växtviloperioden i SWAT, kalibrering med ETa i stället för flöde och en jordartskarta med högre upplösning samt fler jordprover.

With a changing climate and an increased competition over water resources for agricultural irrigation, the need to improve crop water productivity using time and cost-efficient methodologies have become critically important. The Malwathu Oya river basin in Sri Lanka is struggling with water scarcity, which threatens food security and the income of farmers. In this study, freely available remote sensed land- and water productivity data from FAO’s WaPOR database was evaluated. The evaluation consisted of a comparison of the WaPOR data and primary collected field data using the crop water model, AquaCrop, for three irrigation schemes in the Malwathu Oya river basin. Additionally, the spatio-temporal variability in crop water use within and across these three irrigation schemes was assessed using indicators derived from the WaPOR portal. The evaluation was conducted for the main cultivation season, called Maha, between 2010 and 2021.  The WaPOR and AquaCrop actual evapotranspiration (ETa) values were found to be in relatively good agreement (312–537 and 400–465 mm respectively). WaPOR yield values (2.5–2.9 ton/ha) were however lower compared to the AquaCrop simulated yield values and historical yield data (4.6–5.7 and 4.4–5.6 ton/ha respectively). Difference in calculation methodology, possible sources of error in WaPOR conversion calculations and limitations in accuracy caused by cloud coverage when collecting satellite data could be explanations for this. Prior knowledge and accurate allocation of the crop type and parameters used in conversion calculations in WaPOR is therefore of significant influence. From the spatio-temporal variation assessment with WaPOR indicators, a fair uniformity of the water distribution within the irrigation schemes was shown (CV 11–19 %). The beneficial water use (BWU) in the irrigation schemes showed lower values (50–90 % allocated to T) for years when the available water amount was higher, which could be explained by the higher rate of water lost through soil evaporation. Crop water productivity (CWP) values showed higher values (about 0.70 kgDM/m3) when the available water amount was higher, indicating that yield production is sensitive to water-scarce environments. Applying a yield boundary function, representing the best attainable yield in relation to water resource, showed that there is potential to achieve the same yield with less amount of water. There are thus possibilities for improved water productivity in the three irrigation schemes investigated. For future research it is recommended to perform a sensitivity analysis for WaPOR and ground truth with yield data to obtain a better understanding of potential limitations. To obtain more precise site descriptions it is also recommended to ground truth AquaCrop with yield and soil data.