Abstract. The Lake Chad Basin, located in the center of North Africa, is characterized by strong climate seasonality with a pronounced short annual precipitation period and high potential evapotranspiration. Groundwater is an essential source for drinking water supply as well as for agriculture and groundwater related ecosystems. Thus, assessment of groundwater recharge is very important although difficult, because of the strong effects of evaporation and transpiration as well as limited available data. A simple, generalized approach, which requires only a small number of field data, freely available remote sensing data as well as well-established concepts and models, is tested for assessing groundwater recharge in the southern part of the basin. This work uses the FAO-dual Kc concept to estimate E and T coefficients at six locations that differ in soil texture, climate, and vegetation conditions. Measured values of soil water content and chloride concentrations along vertical soil profiles together with different scenarios for E and T partitioning and a Bayesian calibration approach are used to numerically simulate water flow and chloride transport. Average groundwater recharge rates and the associated model uncertainty at the six locations are assessed for the 2003–2016 time-period. Model results show that interannual variability of groundwater recharge is generally greater than the uncertainty of the modelled groundwater recharge. Furthermore, the soil moisture dynamics at all locations are limited rather by water availability for evaporation in the uppermost part of the soil and by water uptake in the root zone than by the reference evapotranspiration.

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<p>Sustainable irrigation water management is expected to accurately meet crop water requirements in order to avoid stress and, consequently, yield reduction, and at the same time avoid losses of water and nutrients due to deep percolation and leaching. Sensors to monitor soil water status and plant water status (in terms of canopy temperature) can help planning irrigation with respect to time and amounts accordingly. The presented study aimed at quantifying and comparing crop water stress of soybeans irrigated by means of different irrigation systems under subhumid conditions.</p><p>The study site was located in Obersiebenbrunn, Lower Austria, about 30 km east of Vienna. The region is characterized by a mean temperature of 10.5°C with increasing trend due to climate change and mean annual precipitation of 550 mm. The investigations covered the vegetation period of soybean in 2018, from planting in April to harvest in September. Measurement data included precipitation, air temperature, relative humidity and wind velocity. The experimental field of 120x120 m<sup>2</sup> has been divided into four sub-areas: a plot of 14x120 m<sup>2</sup> with drip irrigation (DI), 14x120 m<sup>2</sup> without irrigation (NI), 36x120 m<sup>2</sup> with sprinkler irrigation (SI), and 56x120 m<sup>2</sup> irrigated with a hose reel boom with nozzles (BI). A total of 128, 187 and 114 mm of water were applied in three irrigation events in the plots DI, SI and BI, respectively. Soil water content was monitored in 10 cm depth (HydraProbe, Stevens Water) and matric potential was monitored in 20, 40 and 60 cm depth (Watermark, Irrometer). Canopy temperature was measured every 15 minutes using infrared thermometers (IRT; SI-411, Apogee Instruments). The IRTs were installed with an inclination of 45° at 1.8 m height above ground. Canopy temperature-based water stress indices for irrigation scheduling have been successfully applied in arid environments, but their use is limited in humid areas due to low vapor pressure deficit (VPD). To quantify stress in our study, the Crop Water Stress Index (CWSI) was calculated for each plot and compared to the index resulting from the Degrees Above Canopy Threshold (DACT) method. Unlike the CWSI, the DACT method does not consider VPD to provide a stress index nor requires clear sky conditions. The purpose of the comparison was to revise an alternative method to the CWSI that can be applied in a humid environment.</p><p>CWSI behaved similar for the four sub-areas. As expected, CWSI ≥ 1 during dry periods (representing severe stress) and it decreased considerably after precipitation or irrigation (representing no stress). The plot with overall lower stress was BI, producing the highest yield of the four plots. Results show that DACT may be a more suitable index since all it requires is canopy temperature values and has strong relationship with soil water measurements. Nevertheless, attention must be paid when defining canopy temperature thresholds. Further investigations include the development and test of a decision support system for irrigation scheduling combining both, plant-based and soil water status indicators for water use efficiency analysis.</p>

A b s t r a c t. Knowledge of crop water requirements and the effects of management practices on the amounts of water used for crop transpiration and that lost through soil evaporation is essential for efficient agricultural water management. Therefore, this study investigated the temporal evolution of weekly evaporation and transpiration rates under varying soil water conditions in a conventionally managed soybean field by partitioning evapotranspiration based on a water and δ 18 O-stable isotope mass balance. The estimated rates were considered in combination with vertical soil water distribution, atmospheric demand (based on crop evapotranspiration), actual evapotranspiration, and the plant development stage. This allowed for the weekly rates to be compared to the current conditions resulting from dry periods, rain or irrigation events, and the extent of the canopy. The range of weekly transpiration/evapotranspiration, from blossom to maturation, was between 0.60 (±0.11) and 0.82 (±0.10). Within this range, transpiration/evapotranspiration shifted depending on the vertical soil water distribution and meteorological conditions. During dry soil surface periods, evaporation dropped to almost zero, whereas a wet surface layer substantially increased evaporation/evapotranspiration, even under a closed canopy. Under given conditions, the application of a few intense irrigations before the drying of the soil surface is recommended.K e y w o r d s: irrigation, eddy covariance, water scarcity, water use, water stable isotopes

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