水土平衡：半干旱環(huán)境中原位不飽和地區(qū)水穩(wěn)定同位素的測(cè)量

摘要：
      在納米比亞Cuvelai–Etosha半干盆地采用液體水同位素分析儀（LGR可調(diào)節(jié)離軸積分腔輸出光譜技術(shù)）及商用土壤氣體探頭測(cè)量土壤水分的穩(wěn)定同位素（氘，²H, oxygen-18, ¹⁸O）。結(jié)果證實(shí)了原位測(cè)量土氣水分穩(wěn)定同位素的可行性。在研究區(qū)域獲得了合理而準(zhǔn)確的高時(shí)空分辨率數(shù)據(jù)。測(cè)量結(jié)果與低溫真空萃取及后腔衰蕩激光光譜學(xué)同位素分析的實(shí)驗(yàn)室數(shù)據(jù)一致。
      在2014年6月-10月連續(xù)兩次野外活動(dòng)經(jīng)過(guò)140次測(cè)量，原位同位素?cái)?shù)據(jù)的漂移和跨度修正后的精度分別為：δ²H：1.8，δ¹⁸O：0.48 ‰ 。使用質(zhì)量檢查標(biāo)準(zhǔn)得到平均測(cè)量準(zhǔn)確結(jié)果分別為δ²H ：5 ，δ¹⁸O ：0.3 ‰。定量同位素剖面深度來(lái)計(jì)算土壤水分平衡。水蒸發(fā)量占總地表水蒸發(fā)量72-92%。降雨后蒸發(fā)量立即降低至35-50%范圍。激光光譜儀的原位系統(tǒng)存在與環(huán)境條件相關(guān)的潛在局限性，可通過(guò)使用溫度調(diào)節(jié)室zui小化。而且使用烘箱預(yù)先干燥的土壤原料，土壤的理化性質(zhì)（即粘土礦物）可能會(huì)使系統(tǒng)適用性受到限制。通過(guò)改良校準(zhǔn)程序以及進(jìn)一步研究影響土壤水分同位素比值的數(shù)據(jù)，可減少原位系統(tǒng)的不確定性，尤其在低含水量條件下。此外，無(wú)法從數(shù)據(jù)推斷出土壤呼吸二氧化碳對(duì)根區(qū)同位素值的影響。

引用文獻(xiàn)：

Gaj, M., Beyer, M., Koeniger, P., Wanke, H., Hamutoko, J., and Himmelsbach, T.: In situ unsaturated zone water stable isotope (²H and ¹⁸O) measurements in semi-arid environments: a soil water balance, Hydrol. Earth Syst. Sci., 20, 715-731, doi:10.5194/hess-20-715-2016, 2016.

【英文】

In situ unsaturated zone water stable isotope (²H and ¹⁸O) measurements in semi-arid environments: a soil water balance

Marcel Gaj^1,2, Matthias Beyer¹, Paul Koeniger¹, Heike Wanke³, Josefina Hamutoko³, and Thomas Himmelsbach^1 1Federal Institue for Geosciences and Natural Resources (BGR), Stilleweg 2, Hanover, Germany
²Chair of Hydrology, Faculty of Environment and Natural Resources, University of Freiburg, Fahnenbergplatz, 79098 Freiburg, Germany
³Department of Geology, University of Namibia (UNAM), Windhoek, Namibia

Received: 01 Apr 2015 – Published in Hydrol. Earth Syst. Sci. Discuss.: 23 Jun 2015
Revised: 22 Dec 2015 – Accepted: 25 Jan 2016 – Published: 17 Feb 2016

Abstract.

Stable isotopes (deuterium, ²H, and oxygen-18, ¹⁸O) of soil water were measured in the field using a liquid water isotope analyzer (tunable off-axis integrated cavity output spectroscope, OA-ICOS, LGR) and commercially available soil gas probes (BGL-30, UMS, Munich) in the semi-arid Cuvelai–Etosha Basin (CEB), Namibia. Results support the applicability of an in situ measurement system for the determination of stable isotopes in soil pore water. High spatial and temporal resolution was achieved in the study area with reasonable accuracy and measurements were in agreement with laboratory-based cryogenic vacuum extraction and subsequent cavity ring-down laser spectroscopic isotope analysis (CRDS, L2120-i, Picarro Inc.). After drift and span correction of the in situ isotope data, precision for over 140 measurements taken during two consecutive field campaigns (June and November 2014) was 1.8 and 0.48 ‰ for δ²H and δ¹⁸O, respectively. Mean measurement trueness is determined using quality check standards and was 5 and 0.3 ‰ for δ²H and δ¹⁸O, respectively. The isotope depth profiles are used quantitatively to calculate a soil water balance. The contribution of transpiration to total evapotranspiration ranged between 72 and 92 %. Shortly after a rain event, the contribution of transpiration was much lower, at 35 to 50 %. Potential limitations of such an in situ system are related to environmental conditions which could be minimized by using a temperature-controlled chamber for the laser spectrometer. Further, the applicability of the system using previously oven-dried soil material might be limited by physicochemical soil properties (i.e., clay minerals). Uncertainty in the in situ system is suggested to be reduced by improving the calibration procedure and further studying fractionation effects influencing the isotope ratios in the soil water, especially at low water contents. Furthermore, the influence of soil-respired CO₂ on isotope values within the root zone could not be deduced from the data.

Citation: Gaj, M., Beyer, M., Koeniger, P., Wanke, H., Hamutoko, J., and Himmelsbach, T.: In situ unsaturated zone water stable isotope (²H and ¹⁸O) measurements in semi-arid environments: a soil water balance, Hydrol. Earth Syst. Sci., 20, 715-731, doi:10.5194/hess-20-715-2016, 2016.