No abstract

The negative relationship between grain size (percentage >2.5 mm) and protein content usually observed in barley grain samples is attributed to the presence of thin grains. The objective of this study was to determine whether, in grain samples from a given environment, thin grains had a different protein content than plump grains. Grain samples from field experiments were analysed for grain yield, size and protein content of the whole sample and of four size fractions within each sample. Grain yield ranged from 1.5 to 6.5 mg ha À1 and grain protein (whole sample) ranged from 6.8 to 13.4 %. Most of the variation observed in protein content was explained by the ratio of nitrogen availability to grain yield. Within a grain sample, thin grains had more protein than plump grains (>2.5 mm) only when the protein content of the whole sample was high, that is, when the grain sample came from an environment with a high relative abundance of nitrogen. The fact that grain samples with low grain size tend to have high protein content is not due to the presence of a high proportion of thin grains, because thin grains do not always have more protein than plump grains.

Rainwater harvesting (RWH) has been essential for the establishment of human settlements in many dry regions of the world that lacked suitable surface or ground water resources. A vast fraction of the South American Dry Chaco ecoregion still relies on RWH to support, not only livestock production, but domestic and industrial uses as well. As a result, water capture and storage infrastructure is widely disseminated throughout the region. In this paper we characterized the most typical RWH systems in two contrastingly developed sub-regions of Dry Chaco, ranging from extensive ranching to intensive beef and dairy production (central Argentina and western Paraguay, respectively). In each sub-region, we quantified RWH systems density, spatial distribution and associations with landscape features; furthermore, we illustrated how the daily dynamic of water stock in a typical RWH system contributes to assess their capture and storage efficiency. We found that randomly distributed low sophisticated RWH systems prevailed in central Argentina, while clustered distributed high sophisticated ones were more common in western Paraguay. RWH systems density was ten times higher in western Paraguay (0.94 vs. 0.098 units/km 2), showing an exponential association with land cleared fraction and proximity to villages. The daily dynamic of water stock of the RWH impoundment showed that water harvest events were exponentially associated with precipitation magnitude events (R 2 = 0.86), while annual water losses were explained by infiltration and evaporation fluxes (59 vs. 41%, respectively). Across both sub-regions, RWH accounts for less than 1% of the annual precipitation, playing a minor role on the regional water balance; however at a local level, they can affect several hydrological fluxes including the onset of groundwater recharge and the mitigation of extreme runoff events.

Rainfall partitioning into interception loss, throughfall and stemflow affects the amount and the spatial heterogeneity of water entering into the soil at the patch scale, strongly controlling net primary productivity of drylands. In this paper, we explored rainfall partitioning and its biophysical controls in Larrea divaricata (jarilla), one of the most abundant shrubs in the Dry Chaco rangelands (Argentina). On average, interception loss, throughfall and stemflow accounted for 9.4, 78.6 and 12.0% of total rainfall, respectively. Interception loss proportion decreased with the increment of rainfall event size and intensity, whereas throughfall proportion showed the opposite pattern. Stemflow proportion increased with the increment of rainfall event size but presented different relations with rainfall event intensity. The increment of rainfall event intensity increased the stemflow in small events (<20 mm), but decreased it in large events (>20 mm). Stemflow increased in plants with higher angles of insertion of stems (measured at 50 and 100 cm from soil surface; p < .05 and p < .01, respectively), but decreased in plants with larger canopy areas (p = .01). Spatial distribution of throughfall (coefficient of variation) decreased with the increment of rainfall event size and intensity. L. divaricata presented more stemflow generation and fewer interception losses than other similar woody species. Our findings help to understand the key role of vegetation canopy affecting the amount of water entering into the soil in drylands.

No abstract

Abstract. Effectively managing net primary productivity in drylands for grazing and other uses depends on understanding how limited rainfall input is redistributed by runoff and runon among vegetation patches, particularly for patches that contrast between lesser and greater amounts of vegetation cover. Due in part to data limitations, ecohydrologists generally have focused on rainfall event size to characterize water redistribution processes. Here we use soil moisture data from a semiarid woodland to highlight how, when event size is controlled and runoff and interception are negligible at the stand scale, rainfall intensity drives the relationship between water redistribution and canopy and soil patch attributes. Horizontal water redistribution variability increased with rainfall intensity and differed between patches with contrasting vegetation cover. Sparsely vegetated patches gained relatively more water during lower intensity events, whereas densely vegetated ones gained relatively more water during higher intensity events. Consequently, range managers need to account for the distribution of rainfall event intensity, as well as event size, to assess the consequences of climate variability and change on net primary productivity. More generally, our results suggest that rainfall intensity needs to be considered in addition to event size to understand vegetation patch dynamics in drylands.

No abstract

Soil evaporation is a dominant water flux of flat dry ecosystems, reducing available water for plant transpiration. Vegetation plays a key role at controlling evaporation, especially by altering soil surface micro‐meteorological conditions. Here, we explored the vegetation cover effect on soil evaporation, differentiating the effects of canopy cover (shadow) and of surface cover (litter) in forests and pastures of Dry Chaco rangelands (San Luis, Argentina). We measured daily soil evaporation using irrigated micro‐lysimeters installed at regularly spaced (2 m) patches along transects in native dry forests (n = 54 patches) and pastures (n = 27 patches). In each forest patch, we established a pair of micro‐lysimeters, one with litter (3 cm depth, representing high litter cover conditions of the site) and one with bare soil, but in pastures, only one micro‐lysimeter with bare soil was installed at each patch (representing the typical no litter cover conditions of pastures of the study site). We found that, when soil water was not limiting, litter cover had the strongest effect in reducing evaporation rates, with a 4‐ and 6.4‐fold reduction respect to bare soil micro‐lysimeters in the forest and pasture, respectively. Evaporation decreased sharply with declining incident radiation fraction in bare soil micro‐lysimeters from 5.6 mm/day (full radiation) to 3.5 mm/day (full canopy shadow; R2 = 0.50). Litter‐covered micro‐lysimeters showed lower and more stable evaporation rates, decreasing only from 1.35 to 1.03 mm/day under the same radiation conditions (R2 = 0.10). In accordance with J.T. Ritchie evaporation model, we identified a threshold of ~10.5 mm of cumulative evaporation at which evaporation switched from energy to water limitation in all situations, as revealed by declining evaporation rates and raising surface temperatures. Under typical wet–summer conditions, the pasture, the forest with bare soil, and the forest with litter would need on average a drying cycle of 1.5, 2.5, and 9.5 days, respectively, to reach that threshold. Simulations showed that, considering the distribution of rainfall events at our study site, litter would maintain evaporation in the energy‐limited mode most of the time (68.8% of summer days), potentially favouring transpiration. The ecohydrological key role of soil litter controlling evaporation highlights the importance of an accurate assessment of management practices controlling the evaporation/transpiration partition in dry ecosystems.