Laurent Ruiz

Modelling the effect of the spatial distribution of agricultural practices on nitrogen fluxes in rural catchments

Abstract An integrated, hydrology and nitrogen dynamics model was developed to study the spatial interactions between soil and groundwater that can affect the nitrogen delivery to streamwater in rural catchments. The hydrological model TNT is based on TOPMODEL hypotheses but is it fully distributed according to a regular square grid. A subsurface flow component was distinguished to account for the supply of groundwater and nitrate to downslope soils. The crop growth and nitrogen biotransformations were simulated using an existing generic crop model, STICS. Both models are process-based, but kept as simple as possible. The integrated model was applied to theoretical catchments to analyze the combined effects of geomorphology and crop distribution on the whole catchment nitrogen budget. The catchments differed both in the slope profile and in the pattern of water pathways. The results suggest that placing crops acting as nitrogen sinks downslope potentially polluting crops could reduce significantly the streamwater contamination by nitrate. This effect is the highest for catchments with parallel water pathways and a wide concave bottomland. Nitrogen uptake by sink crops was quantitatively more important than denitrification to reduce nitrogen output. It is concluded that this model, although still in development, may prove an interesting working tool to investigate the effect of the landscape structure on nutrient budgets in ecosystems.

Process consistency in models: The importance of system signatures, expert knowledge, and process complexity

Hydrological models frequently suffer from limited predictive power despite adequate calibration performances. This can indicate insufficient representations of the underlying processes. Thus, ways are sought to increase model consistency while satisfying the contrasting priorities of increased model complexity and limited equifinality. In this study, the value of a systematic use of hydrological signatures and expert knowledge for increasing model consistency was tested. It was found that a simple conceptual model, constrained by four calibration objective functions, was able to adequately reproduce the hydrograph in the calibration period. The model, however, could not reproduce a suite of hydrological signatures, indicating a lack of model consistency. Subsequently, testing 11 models, model complexity was increased in a stepwise way and counter-balanced by “prior constraints,” inferred from expert knowledge to ensure a model which behaves well with respect to the modelers perception of the system. We showed that, in spite of unchanged calibration performance, the most complex model setup exhibited increased performance in the independent test period and skill to better reproduce all tested signatures, indicating a better system representation. The results suggest that a model may be inadequate despite good performance with respect to multiple calibration objectives and that increasing model complexity, if counter-balanced by prior constraints, can significantly increase predictive performance of a model and its skill to reproduce hydrological signatures. The results strongly illustrate the need to balance automated model calibration with a more expert-knowledge-driven strategy of constraining models.

pH mapping in transparent gel using color indicator videodensitometry

The colored pH indicator method introduced by Weisenseel et al. (1979) is particularly useful for localizing the zones along roots where acidification/alkalinization occurs. It can also be used to assess the direction and intensity of the proton fluxes. Because the method has not been quantitatively evaluated, however, it is nowadays little used or used in conjunction with other such as potentiometry. In the present study we examine the theoretical basis underlying this method of colorimetric visualization and show its similarity to spectrodensitometry. It thus becomes possible to quantify the luminous information and express it in terms of environmental pH. We describe the method used, emphasizing in particular the conditions required to achieve maximum accuracy of measurement, and an appropriate experimental device. pH distribution around roots can be mapped with a relative error of 0.03 pH units. The experimental device is easy to use and incorporates a computer-controlled video camera, thanks to which al acquisition and calculation procedures can be automated.

The role of climate on inter-annual variation in stream nitrate fluxes and concentrations

In recent decades, temporal variations in nitrate fluxes and concentrations in temperate rivers have resulted from the interaction of anthropogenic and climatic factors. The effect of climatic drivers remains unclear, while the relative importance of the drivers seems to be highly site dependent. This paper focuses on 2-6 year variations called meso-scale variations, and analyses the climatic drivers of these variations in a study site characterized by high N inputs from intensive animal farming systems and shallow aquifers with impervious bedrock in a temperate climate. Three approaches are developed: 1) an analysis of long-term records of nitrate fluxes and nitrate concentrations in 30 coastal rivers of Western France, which were well-marked by meso-scale cycles in the fluxes and concentration with a slight hysteresis; 2) a test of the climatic control using a lumped two-box model, which demonstrates that hydrological assumptions are sufficient to explain these meso-scale cycles; and 3) a model of nitrate fluxes and concentrations in two contrasted catchments subjected to recent mitigation measures, which analyses nitrate fluxes and concentrations in relation to N stored in groundwater. In coastal rivers, hydrological drivers (i.e., effective rainfall), and particularly the dynamics of the water table and rather stable nitrate concentration, explain the meso-scale cyclic patterns. In the headwater catchment, agricultural and hydrological drivers can interact according to their settings. The requirements to better distinguish the effect of climate and human changes in integrated water management are addressed: long-term monitoring, coupling the analysis and the modelling of large sets of catchments incorporating different sizes, land uses and environmental factors.

Long-Term Effects of High Nitrogen Loads on Cation and Carbon Riverine Export in Agricultural Catchments

The intensification of agriculture in recent decades has resulted in extremely high nitrogen inputs to ecosystems. One effect has been H(+) release through NH(4)(+) oxidation in soils, which increases rock weathering and leads to acidification processes such as base-cation leaching from the soil exchange complex. This study investigated the evolution of cation concentrations over the past 50 years in rivers from the Armorican crystalline shield (Brittany, western France). On a regional scale, acidification has resulted in increased base-cation riverine exports (Ca(2+), Mg(2+), Na(+), K(+)) correlated with the increased NO(3)(-) concentration. The estimated cation increase is 0.7 mmol(+)/L for Ca(2+) + Mg(2+) and 0.85 mmol(+)/L for total cations. According to mass balance, cation loss represents >30% of the base-cation exchange capacity of soils. Long-term acidification thus contributes to a decline in soil productivity. Estimates of the total organic nitrogen annually produced worldwide indicate that acidification may also constitute an additional carbon source in crystalline catchments if compensated by liming practices.

Modeling the potential benefits of catch-crop introduction in fodder crop rotations in a Western Europe landscape

Among possible mitigation options to reduce agricultural-borne nitrate fluxes to water bodies, introduction of catch crop before spring crops is acknowledged as a cost-efficient solution at the plot scale, but it was rarely assessed at the catchment level. This study aims to evaluate a set of catch crop implantation scenarios and their consequences in a coastal catchment prone to eutrophication. The objectives are (i) to discuss the potential benefits of catch crop introduction taking into account the limitations due to the physiographic and agricultural context of the area (ii) to propose a multicriteria classification of these scenarios as a basis for discussion with stakeholders. We used the distributed agro-hydrological model TNT2 to simulate 25 scenarios of catch crop management, differing in length of catch crop growing period, place in the crop rotation and residue management. The scenarios were classified considering the variations in main crop yields and either nitrogen fluxes in stream or the global nitrogen mass balance at the catchment level. The simulations showed that in the catchment studied, little improvement can be expected from increasing the catch crop surface. Catch crop cultivation was always beneficial to reduce nitrogen losses, but led to adverse effects on main crop yields in some cases. Among the scenarios involving additional catch crop surface, introducing catch crop between two winter cereals appeared as the most promising. The classification of scenarios depended on the chosen criteria: when considering only the reduction of nitrogen fluxes in streams, exporting catch crop residues was the most efficient while when considering the global nitrogen mass balance, soil incorporation of catch crop residues was the most beneficial. This work highlights the interest, while using integrated models, of assessing simulated scenarios with multicriteria approach to provide stakeholder with a picture as complete as possible of the consequences of prospective policies.

‘I am an Intensive Guy’: The Possibility and Conditions of Reconciliation Through the Ecological Intensification Framework

The need for better conciliation between food production and environmental protection calls for new conceptual approaches in agronomy. Ecological intensification (EI) is one of the most encouraging and successful conceptual frameworks for designing more sustainable agricultural systems, though relying upon semantic ambivalences and epistemic tensions. This article discusses abilities and limits of the EI framework in the context of strong social and environmental pressure for agricultural transition. The purpose is thus to put EI at stake in the light of the results of an interdisciplinary and participatory research project that explicitly adopted EI goals in livestock semi-industrialized farming systems. Is it possible to maintain livestock production systems that are simultaneously productive, sustainable, and viable and have low nitrate emissions in vulnerable coastal areas? If so, how do local stakeholders use these approaches? The main steps of the innovation process are described. The effects of political and social dynamics on the continuity of the transition process are analyzed, with a reflexive approach. This experiment invites one to consider that making EI operational in a context of socio-technical transition toward agroecology represents system innovation, requiring on-going dialogue, reflexivity, and long-term involvement by researchers.

The roots of the drought: Hydrology and water uptake strategies mediate forest‐wide demographic response to precipitation

1. Drought-induced tree mortality is expected to increase globally due to climate change, with profound implications for forest composition, function and global climate feedbacks. How drought is experienced by different species is thought to depend fundamentally on where they access water vertically below-ground, but this remains untracked so far due to the difficulty of measuring water availability at depths at which plants access water (few to several tens of metres), the broad temporal scales at which droughts at those depths unfold (seasonal to decadal), and the difficulty in linking these patterns to forest-wide species-specific demographic responses. 2. We address this problem through a new eco-hydrological framework: we used a hydrological model to estimate below-ground water availability by depth over a period of two decades that included a multi-year drought. Given this water availability scenario and 20year long-records of species-specific growth patterns, we inversely estimated the relative depths at which 12 common species in the forest accessed water via a model of water stress. Finally, we tested whether our estimates of species relative uptake depths predicted mortality in the multi-year drought. 3. The hydrological model revealed clear below-ground niches as precipitation was decoupled from water availability by depth at multi-annual scale. Species partitioned the hydrological niche by diverging in their uptake depths and so in the same forest stand, different species experienced very different drought patterns, resulting in clear differences in species-specific growth. Finally, species relative water uptake depths predicted species mortality patterns after the multi-year drought. Species that our method ranked as relying on deeper water were the ones that had suffered from greater mortality, as the zone from which they access water took longer to recharge after depletion. 4. Synthesis. This research changes our understanding of how hydrological niches operate for trees, with a trade-off between realized growth potential and survival under drought with decadal scale return time. The eco-hydrological framework highlights the importance of species-specific below-ground strategies in predicting forest response to drought. Applying this framework more broadly may help us better understand species coexistence in diverse forest communities and improve mechanistic predictions of forests productivity and compositional change under future climate.

REY-Th-U Solute Dynamics in the Critical Zone: Combined Influence of Chemical Weathering, Atmospheric Deposit Leaching and Vegetation Cycling (Mule Hole Watershed, South India)

The source and proportion of REY, Th and U exported by groundwater and by the ephemeral stream along with the elemental proportions passing through vegetation have been assessed in the sub-humid tropical forested CZO of Mule Hole, Southern India. The study relies on a pluri-annual hydro-geochemical monitoring combined with a hydrological model. The significant difference between the soil input (SI) and output (SO) solute fluxes (mmol/km2/yr) of LREE (SI-SO = 13250-1500), HREE (1930-235), Th (64-12) and U (63-25) indicates a strong uptake by roots carried by canopy and forest floor processes. The contribution of atmospheric dust leaching can reach about 60% of LREE and 80% of HREE. At the watershed scale, the U solute flux exported by groundwater (180 mmol/km2/yr) mainly originates from the breakdown of primary U-bearing accessory minerals and dominates by a factor of 25 the stream flux. The precipitation of authigenic U bearing phases and adsorption onto Fe-oxides and oxyhydroxides plays a significant role for limiting the U mobility. In the groundwater, the plagioclase chemical weathering is efficiently traced by the positive Eu-anomaly. The very low (REY) to nil (Th) contents are explained by the precipitation of authigenic phases. In the stream flow, dominated by the overland flow (87% of the yearly stream flow), the solute exports (in mmol/km2/yr) of REY (1080 for LREE and 160 for HREE) and of Th (14) dominate those by groundwater. Their mobility is enhanced by chelation with organic ligands produced by forest floor and canopy processes.

Irrigation History Estimation Using Multitemporal Landsat Satellite Images: Application to an Intensive Groundwater Irrigated Agricultural Watershed in India

Groundwater has rapidly evolved as a primary source for irrigation in Indian agriculture. Over-exploitation of the groundwater substantially depletes the natural water table and has negative impacts on the water resource availability. The overarching goal of the proposed research is to identify the historical evolution of irrigated cropland for the post-monsoon (rabi) and summer cropping seasons in the Berambadi watershed (Area = 89 km2) of Kabini River basin, southern India. Approximately five-year interval irrigated area maps were generated using 30 m spatial resolution Landsat satellite images for the period from 1990 to 2016. The potential of Support Vector Machine (SVM) was assessed to discriminate irrigated and non-irrigated croplands. Three indices, Normalized Difference Vegetation Index (NDVI), Normalized Difference Moisture Index (NDMI) and Enhanced Vegetation Index (EVI), were derived from multi-temporal Landsat satellite images. Spatially distributed intensive ground observations were collected for training and validation of the SVM models. The irrigated and non-irrigated croplands were estimated with high classification accuracy (kappa coefficient greater than 0.9). At the watershed scale, this approach allowed highlighting the contrasted evolution of multiple-cropping (two successive crops in rabi and summer seasons that often imply dual irrigation) with a steady increase in the upstream and a recent decrease in the downstream of the watershed. Moreover, the multiple-cropping was found to be much more frequent in the valleys. These intensive practices were found to have significant impacts on the water resources, with a drastic decline in the water table level (more than 50 m). It also impacted the ecosystem: Groundwater level decline was more pronounced in the valleys and the rivers are no more fed by the base flow.