Tunisia has a long history of coping with water scarcity, and the quantification of climate change impacts on runoff is important for future water management. A major requirement for such studies is an estimation of potential evapotranspiration (PET), which is challenging as many regions often lack the observational data needed for physically based PET equations. In this study, different PET estimation approaches were used to study the impact of PET estimation on discharge projections for catchments in Northern Tunisia. Discharge was simulated for five catchments using three rainfall-runoff models (RRMs): HBV, GR4 and IHACRES. A general differential split sample test (GDSST) was used for an RRM robustness evaluation based on subperiods with contrasting climatic conditions for the 1970–2000 period. Three cases with varying PET were considered: (1) daily calculated PET, (2) long-term daily mean PET with the same values for calibration and validation periods (calculated over the calibration period) and (3) long-term daily mean PET varying between calibration and validation periods (calculated over the calibration and validation period separately). Over the historical period, the comparison between cases 1 and 3 showed little impact of reduced PET information on the RRM performance and robustness. The comparison of cases 2 and 3 indicated a limited impact of varying PET between calibration and validation on the RRM results. The impact of varying levels of PET information on hydrological projections was also analysed over two future 30-year periods: mid-term period (2040–2070) and long-term period (2070–2100), with two representative concentration pathway scenarios (RCPs 4.5 and 8.5), by comparing cases 1 and 2. The projected discharge with constant PET (case 2) was generally lower than the projected discharge with variable PET (case 1) but the difference in volume change did not exceed 9% for both the time period and the RCP scenario considered. While PET slightly increased under the different climate change scenarios, actual evapotranspiration (AET) was found to decrease. These opposite trends of PET and AET can be attributed to the projected decrease in precipitation. Overall, our results demonstrate that discharge, in semi-arid regions like Northern Tunisia, is not sensitive to PET estimates since AET is mainly controlled by the availability of soil moisture. This finding is useful for performing studies of climate change impact on hydrological cycles in arid regions, as our study shows that simple PET estimation is a valid approach for such studies.

中文翻译：

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突尼斯北部流量预测对潜在蒸散量估算的敏感性

突尼斯应对水资源短缺的历史悠久，量化气候变化对径流的影响对于未来的水资源管理至关重要。此类研究的主要要求是对潜在蒸散量（PET）的估算，因为许多地区通常缺乏基于物理的PET方程式所需的观测数据，因此这很有挑战性。在这项研究中，使用了不同的PET估算方法来研究PET估算对突尼斯北部集水区流量预测的影响。使用HBV，GR4和IHACRES三种降雨径流模型（RRM）对五个流域的排放进行了模拟。基于1970-2000年期间气候条件不同的子时段，使用通用差分拆分样本测试（GDSST）进行RRM鲁棒性评估。考虑了三种不同PET的病例：（1）每日计算的PET，（2）校准和验证期间具有相同值的长期日均PET（在校准期内计算），（3）校准和验证期间之间的长期日均PET（已计算）分别在校准和验证期内）。在历史时期内，案例1和案例3之间的比较显示，减少PET信息对RRM性能和鲁棒性的影响很小。案例2和案例3的比较表明，在校准和验证之间，不同PET对RRM结果的影响有限。还分析了未来30年的两个阶段：中期（2040-2070）和长期（2070-2100），以及两种代表性的浓缩途径情景（RCP），分析了不同水平的PET信息对水文预测的影响。 4.5和8.5），通过比较案例1和2。恒定PET的预计排放量（案例2）通常低于可变PET的预计排放量（案例1），但在时间段和RCP上体积变化的差异均不超过9％考虑的情况。在不同的气候变化情景下，PET略有增加，但实际蒸散量（AET）却有所减少。PET和AET的这些相反趋势可归因于预计的降水减少。总的来说，我们的结果表明，在像突尼斯北部这样的半干旱地区，排放对PET估计值并不敏感，因为AET主要受土壤水分供应的控制。这一发现对于进行气候变化对干旱地区水文循环影响的研究非常有用，