Search

Abstract Reliable attribution of changes in streamflow is fundamental to our understanding of the hydrological cycle and is needed to enable decision makers to manage water resources in a sustainable way. Here, we used a new attribution method based on the Budyko framework (complementary method) to quantify the contributions of climate change and human activities to the changes in annual streamflow in 22 catchments on China's Loess Plateau during the past three decades. Our results showed that after the Grain-for-Green (GFG) project, the annual streamflow decreased by 36% on average (3–72%), with reductions being more intense in northern catchments. The sensitivity of streamflow to precipitation and potential evapotranspiration also decreased, with a mean rate of −0.7 mm yr−1/mm yr−1 and −0.2 mm yr−1/mm yr−1, respectively. Using the upper and lower bounds of the human effects on streamflow from the complementary method as a reference, we found that these effects at half of the stations were under- or over-estimated by the total differential method. The contribution analysis from the complementary method showed that although human activities decreased streamflow by 26% (or 54% as a relative value) on average, the contribution of potential evapotranspiration alone to the decrease in streamflow was 9% (42%), highlighting the important role of increasing atmospheric moisture demand in the water cycle. In addition, the 5-year incremental analysis showed that the impacts of climate and human activities on streamflow had strong spatiotemporal variability. Graphical abstract Unlabelled Image Highlights • Annual streamflow significantly decreased by 36% on average after the GFG project. • Half of the stations in estimating human effect are underestimated or overestimated by the total differential method. • Potential evapotranspiration alone decreased streamflow by 9% (or 42% as relative value). [ABSTRACT FROM AUTHOR]

Abstract Remote sensing based vegetation index provides a practical method for the monitoring of vegetation dynamics at regional and global scales. Here, using a long-term remotely sensed normalized difference vegetation index (NDVI) dataset, we quantified the vegetation changes in the Loess Plateau (LP) over the last three decades (1982–2015), which includes the period before the Chinese"Grain for Green Program"(GGP) was launched (1982–1999), and the period after the GGP (1999–2015). The correlations between the NDVI and four climate related variables, i.e., precipitation, temperature, root-soil moisture (RSM), and a drought proxy-standardized evapotranspiration deficit index (SEDI), were also examined. The results indicated that, (i) the GGP strongly changed the vegetation in the LP. The growing-season mean NDVI (GSM-NDVI) and the annual mean NDVI (AM-NDVI) decreased slightly before the GGP launched in 1999, with slopes of −3.38 × 10−3 and − 8.00 × 10−4 year−1, respectively. However, they showed slight and significant (p < 0.05) increases after the GGP, with slopes of 4.75 × 10−3 and 2.32 × 10−3 year−1, respectively. (ii) Climate change (i.e., warming and drying) resulted in adverse effects on vegetation in the LP during the period before the GGP. However, the observed changes (i.e., wetting and reduced drought) exerted positive effects on the vegetation during the period after the GGP. (iii) Inter-annual variations of spatially averaged NDVI over the LP were primarily determined by RSM rather than any other climate related variables. In the southeastern LP, the inter-annual variation of GSM-NDVI was mainly determined by precipitation and SEDI, while the inter-annual variation of AM-NDVI was mainly caused by SEDI and RSM. Inter-annual variations of both GSM-NDVI and AM-NDVI were mainly determined by SEDI and RSM in the northwestern LP, and by temperature in the southwestern LP. Graphical abstract Unlabelled Image Highlights • The Grain for Green Program (GGP) greatly changed vegetation in the Loess Plateau. • Climate changes exerted adverse effects on vegetation in the LP before the GGP. • Wetting and reduced drought positively affected vegetation in the LP after the GGP. • The inter-annual variation of NDVI was primarily determined by soil moisture. [ABSTRACT FROM AUTHOR]

Agricultural water resource, mainly consumed through evapotranspiration, plays a critical role in agricultural production of arid and semiarid regions. Quantifying the changes of evapotranspiration in cropland (ETc), and its driving factors, may provide rich information for improving human land-use and water resource management. Here we first investigated the multi-year (2000–2015) changes in the ETc (mm yr−1) and associated driving factors of the Loess Plateau (LP), using a combination of the Vegetation Interfaces Processes model and a factorial analysis of variance. We found that the ETc of the LP showed a significant upward trend of 0.31 km3 yr−2 (3.33 mm yr−2) (p <.05) over the last 16 years, mainly driven by cropland changes (3.77% per year), which combined the contribution of cropland area changes and cropland leaf area index (LAIc) changes. We then examined the changes of the dominant driving factor: cropland, and results indicated that the cropland changes consisted of the decrease in cropland area (net decrease of 10.50 × 103 km2) and the increase in LAIc (increased by 10.72%), which suggest the actual contribution of the ETc uptrend was the increasing LAIc. Our further analysis on the causes of the increasing LAIc by correlating the LAIc with land-use management factors revealed that the cropland greening on the LP showed high positive correlations with the increasing inputs of total power of agriculture machinery and farm plastic film, followed by chemical fertilizer. The increase of LAIc was also promoted by the increased ratio of the garden fruits output to total crops output (increased by 67.12%) and multiple cropping (increased by 21.66%). These results suggest that the ETc uptrend can be related to the agricultural intensification. Our study highlights the need for a realistic representation of socio-economic development and human land-use practices in the sustainable optimal allocation of agricultural water resources on the LP. Unlabelled Image • ET in cropland (ETc) showed upward trend over the Loess Plateau from 2000 to 2015. • Land-use optimization drives cropland greening and promotes multi-year rise of ETc. • Socioeconomic development and land-use practice affect agricultural water allocation. [ABSTRACT FROM AUTHOR]