Accurate evaluation of evapotranspiration (ET) flux is an important issue in sustainable urban drainage systems that target not only flow rate limitations, but also aim at the restoration of natural water balances. This is especially true in context where infiltration possibilities are limited. However, its assessment suffers from insufficient understanding. In this study, ET in 1 m3 pilot rain gardens were studied from eight lysimeters monitored for three years in Paris (France). Daily ET was calculated for each lysimeter based on a mass balance approach and the related uncertainties were assessed at ±0.42 to 0.58 mm. Results showed that for these lysimeters, ET is the major term in water budget (61 to 90% of the precipitations) with maximum values reaching 8–12 mm. Furthermore, the major determinants of ET are the existence or not of an internal water storage and the atmospheric factors. The vegetation type is a secondary determinant, with little difference between herbaceous and shrub configurations, maximum ET for spontaneous vegetation, and minimal values when vegetation was regularly removed. Shading of lysimeters by surroundings buildings is also important, leading to lower values. Finally, ET of lysimeters is higher than tested reference values (evaporimeter, FAO-56, and local Météo-France equations).

<p>Sustainable urban drainage systems aim to promote the infiltration and the evapotranspiration (ET) processes rather than the runoff. In this study, the ET in 1 m<sup>3</sup> pilot stormwater planters were studied from eight lysimeters monitored for three years in a dense urban environment in Paris (France). In each lysimeter, a piezometer, four weighing cells and a tipping bucket are used to measure respectively the water level in the internal water storage (IWS), the mass change of the whole lysimeter and the underdrain flow. Meteorological data, precipitation and water level are also collected respectively from the weather station, the rain gauge and the pan evaporimeter installed next to the lysimeters.</p><p>Daily ET was calculated for each lysimeter based on a mass balance approach. The uncertainties related to the daily ET estimates were assessed at ± 0.42 to 0.58 mm depending to the lysimeter and according to the uncertainty propagation law. Results showed that for these lysimeters, with an impluvium equal to 4 times the vegetated surface, ET is the major term in water budget (57 to 90% of the cumulated water inputs) with maximum daily values reaching 8 mm/d. In addition, the observations showed that the major determinants of ET are the existence or not of an internal water storage (IWS) and the atmospheric factors (global radiation, air temperature and in a minor extent air humidity). The type of vegetation is a secondary determinant, with little difference between the herbaceous and the shrub configurations, maximum ET for spontaneous vegetation and minimal values when the vegetation was regularly removed. Shading of lysimeters by surroundings buildings is also an important factor and leads to lower values. Finally, ET with an IWS is higher than reference values tested (evaporimeter, FAO-56, and local Météo-France equations), except for regional Météo-France formula which overestimates ET of lysimeters and especially in summer. For future studies, it is expected to include some aspects in the experiments for explicitly addressing shading effects and vegetation evolution.</p>

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