Fluvial geomorphological dynamics and land use changes: impact on the organic carbon stocks of soil and sediment

The drainage basin of the Turrilla river (SE of Spain) went through important land cover changes since 1950s; from mainly an agrarian scenario in 1956 to other depopulated and forested in 2015. This study analyzes the effects of land use changes on fluvial dynamics and their relationship with the organic carbon (OC) stock in fluvial sedimentary deposits as well as in the soil of the basin. Methods included a fluvial geomorphological analysis and a land use change analysis in combination with OC databases of soil and sediment. Results showed that the fluvial channel experienced important morphological changes related to different erosion processes and stabilization of fluvial deposits, induced by land use changes in the drainage area. The active channel decreased 63% in the study period, while bank erosion and gully erosion increased (34% and 21 %, respectively). Alluvial fans and alluvial plain were also extended (21% and 7 %, respectively) and alluvial bars were colonized by vegetation. Sediment was impoverished in OC compared to catchment soils (0.24 enrichment ratio sediment/soil). However the increase of OC stock (Mg ha-1) was very similar between soil (25 %) and sediment (23 %). The total reservoir of OC (Mg) increased 27% in sediments and 25% in the catchment soils. Results show the large influence of geomorphological dynamics on the OC reservoir at the catchment scale. A very high potential of fluvial sediments to increase OC sinks was observed, particularly in scenarios where the active channel is narrowed and the fluvial channel is encroached with vegetation, facilitating the input of OC in sediment. The potential of sediment to sequester organic carbon could be very useful in planning and management of fluvial sedimentary zones in climate change mitigation policies. © 2019, Universidad Austral de Chile. All rights reserved.Este estudio ha recibido apoyo financiero del proyecto DISECO (CGL2014-55405-R) del Plan Nacional de Ciencia del Ministerio de Economía y Competitividad de España, del proyecto SOGLO (P7/24 IAP BELSPO) del gobierno de Bélgica. AHM recibió apoyo financiero para una estancia en la Universidad Nacional de Córdoba (Argentina) del Banco de Santander mediante el Convenio Becas de Intercambio Latinoamérica (Programa ILA). CBF recibió apoyo financiero para dos estancias en el extranjero del programa Salvador de Madariaga 2017 (Ministerio de Educación, Cultura y Deporte, Gobierno de España) y del programa Jiménez de la Espada 2017 (Fundación Séneca, Agencia de Ciencia y Tecnología de la Región de Murcia). MAB fue parcialmente financiada por un contrato Juan de la Cierva-Incorporación (Ref: IJCI-2015-23500). Todas estas estancias permitieron el trabajo continuado en la redacción de este artículo

Evaluation of long-term changes in precipitation over Bolivia based on observations and Coupled Model Intercomparison Project models

Using observations and model simulations from the 5th and 6th phases of the Coupled Model Intercomparison Project (CMIP5 and CMIP6, respectively), this study evaluated changes in monthly, seasonal, and annual precipitation over Bolivia from 1950 to 2019. Results demonstrate that observed precipitation is characterized by strong interannual and decadal variability. However, long-term precipitation trends were not identified on the annual scale. Similarly, changes in seasonal precipitation were almost nonsignificant (p > .05) for the study period. Spatially, albeit with its complex orography, no substantial regional variations in observed precipitation trends can be identified across Bolivia. In contrast, long-term precipitation trends, based on CMIP5 and CMIP6 models, suggest a dominance of negative trends, mainly during austral winter (JJA) (−10%) and spring (SON) (−15%). These negative trends were more pronounced in the lowlands of Bolivia (−20%). Overall, these contradictory results highlight the need for validating precipitation trend outputs from model simulations, especially in areas of complex topography like Bolivia.This work was sup-ported by the research projects CGL2017-82216-R,PCI2019-103631, and PID2019-108589RA-I00, financedby the Spanish Ministry of Science and FEDER, theCROSSDRO project financed by the AXIS (Assessment ofCross(X)-sectorial climate impacts and pathways for Sus-tainable transformation), the JPI-Climate co-funded callof the European Commission, and the LINCGLOBAL-CSIC project (INCGLO0023, RED-CLIMA)

Assessment of vapor pressure deficit variability and trends in Spain and possible connections with soil moisture

The Vapor Pressure Deficit (VPD) is one of the most relevant surface meteorological variables; with important implications in ecology, hydrology, and atmosphere. By understanding the processes involved in the variability and trend of the VPD, it is possible to assess the possible impacts and implications related to both physical and human environments, like plant function, water use efficiency, net ecosystem production, atmospheric CO2 growth rate, etc. This study analysed recent temporal variability and trends in VPD in Spain between 1980 and 2020 using a recently developed high-quality dataset. Also, the connection between VPD and soil moisture and other key climate variables (e.g. air temperature, precipitation, and relative humidity) was assessed on different time scales varying from weekly to annual. The objective was to determine if changes in land-atmosphere feedbacks connected with soil moisture and evapotranspiration anomalies have been relevant to assess the interannual variability and trends in VPD. Results demonstrate that VPD exhibited a clear seasonality and dominant positive trends on both the seasonal (mainly spring and summer) and annual scales. Rather, trends were statistically non-significant (p > 0.05) during winter and autumn. Spatially, VPD positive trends were more pronounced in southern and eastern of Spain. Also, results suggest that recent trends of VPD shows low contribution of variables that drive land-atmosphere feedbacks (e.g. evapotranspiration, and soil moisture) in comparison to the role of global warming processes. Notably, the variability of VPD seems to be less coupled with soil moisture variability during summertime, while it is better interrelated during winter, indicating that VPD variability would be mostly related to climate variability mechanisms that control temperature and relative humidity than to land-atmosohere feedbacks. Overall, our findings highlight the importance of assessing driving forces and physical mechanisms that control VPD variability using high-quality climate datasets, especially, in semiarid and sub-humid regions of the world

Conexión entre los patrones de circulación atmosférica del Atlántico Norte y la variabilidad de la precipitación invernal en la Península Ibérica

Las proyecciones climáticas más avanzadas indican, con alta confianza, un incremento de la frecuencia, persistencia y severidad de los periodos secos en el sur de Europa para finales del Siglo XXI. Esa intensificación de las sequías se explica por el descenso de la precipitación, y especialmente por el incremento de la temperatura y ratio de evapotranspiración. En lo que refiere a los cambios de precipitación, en términos absolutos, la mayor reducción es proyectada en invierno. La expansión del cinturón de altas presiones subtropical es proyectada junto al desplazamiento de las tormentas hacia el norte de Europa, donde se proyectan condiciones más húmedas a finales del Siglo XXI. Este patrón de cambios esperados en Europa parece estar siendo observado durante las últimas décadas. Focalizando en la Península Ibérica, una mayor frecuencia e intensidad de los periodos de sequía desde 1980 corrobora la tendencia proyectada por simulaciones. No obstante, esa señal parece ser no tan robusta cuando se revisan periodos más largos de la Era Instrumental. Paralelamente, las proyecciones de precipitaciones continúan mostrando alta incertidumbre entre modelos y simulaciones de cada uno de ellos. Surge por tanto la necesidad de comprender óptimamente la variabilidad climática observada en el largo plazo, así como en qué factores radica la incertidumbre de las proyecciones climáticas. Bajo ese objetivo principal, esta tesis doctoral intenta aumentar el conocimiento sobre la variabilidad observada de la precipitación en Europa, fundamentalmente en la Península Ibérica, desde el origen de los registros disponibles. Conectando su comportamiento con la dinámica de los modos de variabilidad climática principales en la región, e.g., la Oscilación del Atlántico Norte (NAO). Para, entonces, evaluar si las proyecciones climáticas son capaces de interpretar toda la variabilidad climática que deriva en ciclos decadales y centenales de la precipitación. Los resultados confirman un comportamiento ciertamente singular en numerosos indicadores climáticos desde 1980. En la Península Ibérica, amplias regiones registran un descenso drástico de las precipitaciones, que se propaga, incluso con más intensidad, en las series hidrológicas. Esta tendencia es conectada principalmente con la intensificación de NAO hacia fases positivas, que se resume en una mayor frecuencia de altas presiones sobre las Azores. Al menos en la Era Industrial, nuestros resultados evidencian su comportamiento anómalo desde 1980 en base a varios indicadores como su rol más predominante, explicando más varianza que en décadas anteriores; siendo más intenso, algo sin precedentes; y mostrando un patrón espacial con ciertas particularidades. Aunque otros factores parecen jugar un papel importante, nuestros hallazgos muestran con alta confianza que el descenso de la precipitación invernal es el desencadenante principal de la intensificación de las sequías desde 1980 en la mayor parte del territorio de la Península Ibérica, y probablemente, en otras regiones del Mediterráneo Occidental. Dado que las proyecciones climáticas prevén la persistencia de la intensificación de NAO+ (e.g., expansión de la Célula de Hadley, intensificación del cinturón subtropical), estas condiciones más secas podrían extenderse durante las próximas décadas, e incluso agravarse, constituyendo una amenaza ambiental clave para la disponibilidad de recursos hídricos. No obstante, la magnitud de los cambios esperados difiere considerablemente entre modelos capaces de reproducir long-term variaciones en la circulación de larga escala, y aquellos modelos, demasiado rígidos, que no pueden. Esto abre un horizonte muy ilusionante en el que mejorar la habilidad de las simulaciones capturando completamente la variabilidad natural observada del sistema climático es una prioridad. Jamás podremos conocer con exactitud que depara al clima a escala regional, y sus impactos a los seres vivos en esos territorios, si los modelos climáticos continúan expresando cualquier sesgo intrínseco al capturar indicadores tan fundamentales como las condiciones sinópticas en el Atlántico Norte.The most advanced climate projections indicate, with high confidence, an increase in the frequency, persistence, and severity of dry periods in southern Europe by the end of the 21st century. This intensification of droughts is explained by a decrease in precipitation, and especially the rise temperatures and evapotranspiration rates. In terms of precipitation changes, the largest reduction is projected for winter. The expansion of the subtropical high-pressure belt is projected, along with the northward movement of storms into northern Europe, where wetter conditions are projected by the end of the 21st century. This pattern of expected changes in Europe appears to have been observed over the last few decades. Focusing on the Iberian Peninsula, a higher frequency and intensity of drought periods since 1980 corroborate the trend projected by simulations. However, this signal seems less robust when examining longer periods of the Instrumental Era. Additionally, precipitation projections continue to exhibit high uncertainty among models and their simulations. Therefore, there is a need to gain a comprehensive understanding of the long-term observed climatic variability, as well as the factors contributing to the uncertainty in climate projections. With this primary objective in mind, this doctoral thesis aims to enhance knowledge about observed precipitation variability in Europe, primarily in the Iberian Peninsula, from the beginning of available records. It seeks to connect this behavior with the dynamics of the major climate variability modes in the region, such as the North Atlantic Oscillation (NAO). The goal is to evaluate whether climate projections can effectively interpret all the climatic variability that results in decadal and centennial precipitation cycles. The results confirm a notably unique behavior in numerous climate indicators since 1980. In the Iberian Peninsula, extensive regions experience a significant decline in precipitation, which propagates, with even greater intensity, in hydrological series. This trend is primarily linked to the intensification of NAO into positive phases, resulting in a higher frequency of high-pressure systems over the Azores. At least during the Industrial Era, our results highlight its anomalous behavior since the 1980s based on several indicators, such as its more dominant role, explaining more variance than in previous decades, being more intense, something unprecedented, and showing a spatial pattern with certain particularities. Although other factors appear to play a significant role, our findings show with high confidence that the decrease in winter precipitation is the primary trigger for the intensification of droughts since 1980 in most of the territory of the Iberian Peninsula, and likely in other regions of the Western Mediterranean. Given that climate projections anticipate the persistence of NAO+ intensification (e.g., expansion of the Hadley Cell, intensification of the subtropical belt), these drier conditions could extend over the next decades, and even worsen, constituting a key environmental threat to water resource availability. However, the magnitude of the expected changes differs considerably between models capable of reproducing long-term variations in large-scale circulation and those models that are too rigid. This opens up an exciting horizon in which improving the ability of simulations to capture fully the observed natural variability of the climatic system is a priority. We will never be able to accurately predict what the climate holds at a regional scale, and its impacts on living beings in those territories, if climate models continue to exhibit inherent biases in capturing such fundamental indicators as synoptic conditions in the North Atlantic

Estudio del índice de aridez bajo escenarios de cambio climático en la cuenca del río Almanzora (Almería)

Ponencia presentada en: XI Congreso de la Asociación Española de Climatología celebrado en Cartagena entre el 17 y el 19 de octubre de 2018.[ES]El empleo de modelos climáticos regionales, concretamente de las proyecciones de precipitación y evapotranspiración de referencia (ET0) han servido para analizar los niveles de disponibilidad hídrica en escenarios de cambio climático en una cuenca de drenaje mediterránea con importantes transformaciones en la cobertura del suelo durante 1956-2007. A través del índice de aridez adoptado por la UNEP (1997) se han evaluado diferentes demandas de agua asociadas a los cambios experimentados en los usos del suelo. Los resultados muestran un descenso importante en la disponibilidad hídrica de la cuenca, especialmente en el escenario A2 (periodo 2071-2099). Este análisis espacial permite analizar las estimaciones en la evolución del cambio climático para diferentes coberturas del suelo, lo que puede ser de gran utilidad en la planificación y gestión del territorio de acuerdo a las recomendaciones IPCC para establecer políticas de mitigación y adaptación al calentamiento global.[EN]The use of the regional climate models, specifically the precipitation and evapotranspiration of reference (ET0) have been useful to analyse the levels of hydric availability at sceneries of climate change in a Mediterranean drainage basin with important changes in the land cover during 1956-2007. By using the aridity index taken by UNEP (1997), different demands of water related to the changes experimented in the land uses have been evaluated. The results show an important decline in the hydric availability of the basin, especially at A2 scenery (2071-2099 period). This spacial analysis enables to analyse the estimates in the evolution of the climate change for different ground covers, which can be very useful for the planning and territory management according to the IPCC recommendations to establish mitigation and adaptation to global warming policies

Presión antrópica sobre cuencas de drenaje en ecosistemas frágiles: variaciones en las existencias (stock) de carbono orgánico asociadas a cambios morfológicos fluviales

Anthropic changes in the drainage area of catchments can influence dominant erosion processes and sediment sources and mobilize specific carbon pools. It also causes changes in the sedimentary dynamics and thus in the fluvial morphology. At the same time fluvial morphologies can create the conditions for stabilizing organic carbon (OC) in sediments by burial, carbon preservation, slowing down mineralization processes, and terrestrial or aquatic plant colonization. All this might have a significant impact on the fluvial carbon sink or sources. This work explores the impact of changes in the drainage area (reforestation, check-dam building, agricultural abandonment) on fluvial morphology and on the sedimentary carbon sink of an arid and erodible catchment. The methodological approach combines cartographic analysis of land use, geomorphological photointerpretation of the channel and slope-channel connections in 1956 and 2016. Furthermore, soil and sediment sampling across the catchment for organic carbon stock determination was carried out. The watershed underwent a drastic transformation of land use from 1956, changing from an agrarian scenario to a forest pattern in 2016. This evolution altered sedimentological dynamics and fluvial morphologies. The active channel was narrowed (52%) whereas bank erosion (77%) and the adjacent gullies (11%) increased. The inner alluvial plain increased up to 31% and alluvial fans up to 37%. In addition, vegetation in the channel increased up to 16%. All this led to an increase of the total OC pool of fluvial sediments (12%), slightly above than the increase of OC total pool in the soils of the catchment (10%). The ratio of the OC stock sediments/soils was ' 0.8, which indicates the large capacity of carbon sequestration of fluvial sediments, with OC stocks larger than those of agricultural soils. It was found that the geomorphological dynamics plays an important role in the OC fluvial flows. In scenarios of channel narrowing and vegetation encroachment of fluvial morphologies, the sediments can stabilize and generate OC sinks. These processes of OC sequestration in dry and ephemeral channels can have a large relevance for various ecosystem services and should be considered in the management of fluvial sedimentary areas. © 2019 John Wiley and Sons Inc.. All rights reserved

High resolution monitoring and probabilistic prediction of meteorological drought in a Mediterranean environment

Drought remains a costly natural disaster, with strong socio-economic and environmental impacts. Skilful seasonal drought forecasts can help to make early decisions. Here we assess the quality of a prototype seasonal forecasting system for a Mediterranean region (peninsular Spain + Balearic Islands) to predict meteorological drought as measured by the standardised precipitation index (SPI). We first show that there is a high agreement between the official data provided by the Spanish Meteorological Agency and the state-of-the art ERA5 reanalysis, building confidence in using these datasets. Thus, since the ERA5 data are provided in near-real time as it is updated on a monthly basis, it can be used to monitor drought evolution. Then, we demonstrate that it is possible to obtain skilful and reliable seasonal drought predictions several months in advance by applying an ensemble-based streamflow prediction system (ESP, an ensemble based on the rearrangement of historical data) using ERA5 data as initial conditions. The results indicate that a statistical persistence-based model could lead to an actionable seasonal drought forecasting skill thus providing the basis for a cheap and fast prototype for drought early warning

Presión antrópica sobre cuencas de drenaje en ecosistemas frágiles: variaciones en las existencias (stock) de carbono orgánico asociadas a cambios morfológicos fluviales

Anthropic changes in the drainage area of catchments can influence dominant erosion processes and sediment sources and mobilize specific carbon pools. It also causes changes in the sedimentary dynamics and thus in the fluvial morphology. At the same time fluvial morphologies can create the conditions for stabilizing organic carbon (OC) in sediments by burial, carbon preservation, slowing down mineralization processes, and terrestrial or aquatic plant colonization. All this might have a significant impact on the fluvial carbon sink or sources. This work explores the impact of changes in the drainage area (reforestation, check-dam building, agricultural abandonment) on fluvial morphology and on the sedimentary carbon sink of an arid and erodible catchment. The methodological approach combines cartographic analysis of land use, geomorphological photointerpretation of the channel and slope-channel connections in 1956 and 2016. Furthermore, soil and sediment sampling across the catchment for organic carbon stock determination was carried out. The watershed underwent a drastic transformation of land use from 1956, changing from an agrarian scenario to a forest pattern in 2016. This evolution altered sedimentological dynamics and fluvial morphologies. The active channel was narrowed (52%) whereas bank erosion (77%) and the adjacent gullies (11%) increased. The inner alluvial plain increased up to 31% and alluvial fans up to 37%. In addition, vegetation in the channel increased up to 16%. All this led to an increase of the total OC pool of fluvial sediments (12%), slightly above than the increase of OC total pool in the soils of the catchment (10%). The ratio of the OC stock sediments/soils was > 0.8, which indicates the large capacity of carbon sequestration of fluvial sediments, with OC stocks larger than those of agricultural soils. It was found that the geomorphological dynamics plays an important role in the OC fluvial flows. In scenarios of channel narrowing and vegetation encroachment of fluvial morphologies, the sediments can stabilize and generate OC sinks. These processes of OC sequestration in dry and ephemeral channels can have a large relevance for various ecosystem services and should be considered in the management of fluvial sedimentary areas.Cambios antrópicos en el área de drenaje de cuencas hidrológicas determinan los procesos de erosión dominante y las fuentes de sedimento, movilizando ciertos reservorios (pools) de carbono. Además, estos cambios en el área de drenaje influyen en la dinámica sedimentaria con consecuencias en la morfología fluvial. Éstas, a su vez, pueden crear las condiciones para estabilizar carbono orgánico (CO) en los sedimentos por enterramiento, preservación de carbono, ralentización de la mineralización y colonización vegetal terrestre o acuática; causando todo ello un efecto importante en el sumidero o fuentes de carbono fluvial. Este trabajo explora el impacto de las alteraciones en el área de drenaje (reforestaciones, construcción de diques, abandono agrícola) en la morfología fluvial y en el sumidero de CO sedimentario, en una cuenca semiárida y de elevada erodibilidad. La aproximación metodológica combina el análisis cartográfico de los usos del suelo, la fotointerpretación geomorfológica del cauce y formas adyacentes en dos escenarios (1956 y 2016); y muestreos recientes de suelos y sedimentos para la determinación de las existencias (stock) de CO. La cobertura del suelo varió de un escenario principalmente agrario hacia un patrón forestal. Esa evolución alteró la dinámica sedimentológica de la cuenca que se tradujo en importantes cambios morfológicos en el cauce, disminuyendo la superficie de cauce activo (52%), incrementando la erosión lateral (77%) y las cárcavas adyacentes (11%), así como llanuras (31%) y abanicos aluviales (37%). Además, el cauce se revegetó ligeramente (16%). Todo ello propició un aumento del reservorio total de CO en los sedimentos fluviales (12%), ligeramente por encima del aumento del reservorio en los suelos de la cuenca (10%). La razón de las existencias (stock) de CO sedimentos/suelos fue >0.8, lo cual nos indica la gran capacidad de secuestro de carbono de los sedimentos, con existencias de CO por encima de los suelos agrícolas. Se constató que la dinámica geomorfológica fluvial juega un papel relevante en los flujos de CO, y en la capacidad de estabilizar sedimentos que generan sumideros de CO, en condiciones de estrechamiento del cauce activo y reverdecimiento del lecho. Estos procesos de secuestro de carbono en lechos secos y efímeros, tienen una gran relevancia para variados servicios ecosistémicos, y deberían ponerse en valor en la gestión de zonas sedimentarias fluviales