Attributing of global evapotranspiration trends based on the Budyko framework

Actual evapotranspiration (ET) is an essential variable in the hydrological process, linking the carbon, water, and energy cycles. Global ET has significantly changed in the warming climate. Although increasing vapour pressure deficit (VPD) due to global warming enhances atmospheric water demand, it remains unclear how the dynamics of ET are affected. In this study, using multiple datasets, we disentangled the relative contributions of precipitation, net radiation, air temperature (T1), VPD, and wind speed on affecting annual ET linear trend using an advanced separation method that considers the Budyko framework. It is found that the precipitation variability dominantly controls global ET in the dry climates, the net radiation has substantial control over ET in the tropical regions, and VPD is impacting ET trends in boreal mid-latitude climate. The critical role of VPD in controlling ET trends is particularly emphasized due to its influence in controlling the land-atmosphere interactions.

Characteristics of radiative and non-radiative energy fluxes over monsoon trough zone

In order to describe behaviour of radiative and non-radiative erergy fluxes in the surface layer, computation of net radiation, sensible, latent and heat soil flux has been done using hourly global radiation, slow response data of MONTBLEX-90 and surface observation of Varanasi and Jodhpur during rainy and non-rainy days in July 1990. Daily and hourly ground temperature is calculated solving one dimensional heat conduction equation and soil heat flux is computed using force restored method .Outgoing Longwave Radiation (OLR) is calculated by Stefan-Boltzrnann law of radiation and the largest diurnal variability was found over dry convective zone. Results show that OLR from the ground lies in the range 473.0-537.6 Wm-2 at Jodhpur and 497.4 -548.4 Wm-2 at Varanasi during generally cloudy day. The dip in OLR is increascd by 10% with increase of relative humidity and cloudiness. Daily mean of the largest downward soil heat flux are found as 206.4 and 269.4 Wm-2 at Varanasi and Jodhpur respectively during cloudy day. About 40-50% of net radiation is imparted to soil heat flux at Varanasi and Jodhpur. Sum of the hourly non- radiative energy fluxes has not been balanced by net radiation while daily cumulative value of the fluxes balances the net radiation during non-rainy day.

Trend Analysis of Reference Evapotranspiration in the São Francisco River Basin, Brazil

An important requisite in any water resources management program is the knowledge of the natural processes that regulate the hydrological cycle, especially for spatial and temporal analyses of these processes. This work aimed to explore spatial and temporal trends of reference evapotranspiration (ETo), evaluate ETo variation dynamics and quantify the contribution of each one of the meteorological variables in ETo calculation in the São Francisco River basin. Daily data of maximum and minimum air temperature, mean relative air humidity, wind speed and net radiation from 101 weather stations, for the period from 1961 to 2015, were used and are part of the network of the National Institute of Meteorology (INMET). Climate trend analysis was performed using the non-parametric Mann-Kendall statistical test and the sensitivity analysis for the Penman-Monteith equation was carried out based on partial derivatives as a function of the key meteorological variables of ETo (air temperature, net radiation, wind speed and actual water vapor pressure). A significant increase (p<0.01) in vapor pressure deficit (VPD) associated with the increase in mean air temperature contributed to increments in ETo along the studied period. The scenario in the entire São Francisco River basin is of increase in the values of the climatic variables, particularly evapotranspiration (85% of basin area with increments) and rainfall (52% of basin area with decrements). Results indicate an increase in ETo variation rates of approximately 2.424 mm year-2, on average, in the entire basin. The energy term represents higher weight in ETo calculation in 81% of the basin territory.

Estimation of the All-Wave All-Sky Land Surface Daily Net Radiation at Mid-Low Latitudes from MODIS Data Based on ERA5 Constraints

The surface all-wave net radiation (Rn) plays an important role in the energy and water cycles, and most studies of Rn estimations have been conducted using satellite data. As one of the most commonly used satellite data sets, Moderate Resolution Imaging Spectroradiometer (MODIS) data have not been widely used for radiation calculations at mid-low latitudes because of its very low revisit frequency. To improve the daily Rn estimation at mid-low latitudes with MODIS data, four models, including three models built with random forest (RF) and different temporal expansion models and one model built with the look-up-table (LUT) method, are used based on comprehensive in situ radiation measurements collected from 340 globally distributed sites, MODIS top-of-atmosphere (TOA) data, and the fifth generation of European Centre for Medium-Range Weather Forecasts Reanalysis 5 (ERA5) data from 2000 to 2017. After validation against the in situ measurements, it was found that the RF model based on the constraint of the daily Rn from ERA5 (an RF-based model with ERA5) performed the best among the four proposed models, with an overall validated root-mean-square error (RMSE) of 21.83 Wm−2, R2 of 0.89, and a bias of 0.2 Wm−2. It also had the best accuracy compared to four existing products (Global LAnd Surface Satellite Data (GLASS), Clouds and the Earth’s Radiant Energy System Edition 4A (CERES4A), ERA5, and FLUXCOM_RS) across various land cover types and different elevation zones. Further analyses illustrated the effectiveness of the model by introducing the daily Rn from ERA5 into a “black box” RF-based model for Rn estimation at the daily scale, which is used as a physical constraint when the available satellite observations are too limited to provide sufficient information (i.e., when the overpass time is less than twice per day) or the sky is overcast. Overall, the newly-proposed RF-based model with ERA5 in this study shows satisfactory performance and has strong potential to be used for long-term accurate daily Rn global mapping at finer spatial resolutions (e.g., 1 km) at mid-low latitudes.

Evapotranspiration measurements in the Brazilian Caatinga dry forest

&lt;p&gt;The Brazilian semiarid region (one million km&amp;#178;) is densely populated (25 million inhabitants), but its natural water availability is low. Despite the fact that evaporative processes are key to the regional water deficit, the actual evapotranspiration of natural environment has rarely been measured, especially in the native Caatinga dry forest. We hereby propose a simple method that demands the monitoring of five meteorological variables: relative humidity, global radiation, canopy and air temperature, as well as wind speed. These values are used to assess leaf energy balance, yielding net radiation (Rn) and actual evapotranspiration (LE). To estimate the actual Caatinga evapotranspiration under natural conditions and in different seasons, the proposed method was applied &lt;em&gt;in situ&lt;/em&gt; during nine months. The application site was the Aiuaba Experimental Basin, situated in an environmental protection area in the North-eastern Brazil. The method provided consistent results when compared with independent measurements, such as atmospheric demand, leaf area, and soil water content variation. Results indicate that the daily average net radiation is 12 MJ m&lt;sup&gt;-&lt;/sup&gt;&amp;#178;. During the dry season, the actual evapotranspiration is very low, with negligible LE/Rn ratio. Contrastingly, in the rainy season, it raises to 6 mm per day, with average LE/Rn ratio equalling 0.89. The results show that the actual evapotranspiration in the Caatinga long-lasting dry season (up to nine months per year) is controlled by the water availability in the soil.&lt;/p&gt;

Energy balance at the land-surface interface

A small scale field experiment was conducted at the Indian Institute of Tropical Meteorology (IITM). Pashan Pune and the energy budget at the land surface interface was studied for clear and cloudy days over bare soil. Using instrumented towers, a net radiometer and soil temperature probe all the components of the energy budget. i.e. the sensible heat flux, latent heat flux, soil heat flux and net radiation were measured directly and the energy balance was computed. It is observed that when considered over the whole day, the energy budget is fairly balanced. As a part of energy budget, the Bowen's ratio is also discussed.

Crop-coefficients of tomato as derived using monolithic weighing type lysimeter in mid hill region of Meghalaya

Crop coefficients (kc) was determined for tomato (Lycopersicon esculentum Mill.) with the help of UMS-GmBH cylindrical field lysimeter of 30 cm diameter and 120 cm deep and Penman-Monteith FAO-56 model. Eight other models viz. Modified Penman Method, Hargreaves equation, Samani-Hargreaves equation, Thornthwaite equation, Solar Radiation Method, Net Radiation Method, Blaney-Criddle Method and Radiation Method were also used for estimation of ET0­ and compared with Penman-Monteith model to find out the accuracy of prediction with limited weather parameters. Scatter plot and paired t-test were used for comparison. Out of all these models, Blaney-Criddle method, Solar and Net Radiation method were found to yield similar results as given by Penman-Monteith model. The values of crop evapo-transpiration (ETc) were varying from 2.54 mm d-1 to 6.70 mm d-1. The crop-coefficients (kc) for three growth stages of tomato viz., initial, mid and maturity were found to be 0.55, 1.07 and 0.78, respectively.