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Hydraulics, specifically the engineering related to river infrastructure, has had always to deal with issues as floods, erosion debilitating important civil infrastructure (bridges, etc.). In recent years it has been shown that there is predictive value for hydraulics in studying and analyzing these phenomena at a reduced scale. For this reason the microscale experimental canal was build at the Hydraulics Laboratory at the Universidad of Concepción, with a second copy for control purposes at the Control Systems Laboratory (LCS). The partial differential equations that govern the process are unnecessarily complex for control purposes. Known literature offers the alternative of a third order linear system, capturing the canal standing wave with a second order system together with an integrator to account for the mass accumulation. However, such a proposal, even when slowing the control to discard the standing wave dynamics is only valid for long canals. The microscale experimental canal does not satisfies that assumption, therefore we propose and adjust a first order structure to the process, which proves adequate for any given operating point selection. Nevertheless, the main three parameters of the plant model (gain, time constant and time delay) vary with the operating point. As a preliminary control approach we consider a plant model selection for a specific operating point as the nominal plant and adjust a PI control using the reaction curve method of Ziegler-Nichols. We compare the previous PI tuning with a predictive PI λ tuning to achieve robustness, and thus better deal (in this preliminary control approach) with the inherent variability of the plant model parameters.

We explore the relationships between environmental factors and vegetative and reproductive phenology in nine dominant perennial plant species in a semi-arid ecosystem in central Mexico. In spite of the highly seasonal environment at the study site, and a relatively long dry period, at least one species was always in some phase of growth or reproduction for the duration of the study (April 1996–June 1998). Two patterns in leafing were observed. Some shrub species maintained their leaves during the entire year, whereas others lost their leaves during the dry season. The cacti Neobuxbaumia tetetzoandCephalocereus columna-trajanishowed slow growth, however they produced areoles throughout the study. Echinocactus platyacanthusceased areolar production during the driest period of the study. Throughout the both years, the dispersion of flowering and fruiting peaks among all the species studied were random. However, these species formed four distinct groups based on reproductive phenology and life form. (1) Shrubs with a general root system (0–1m depth), Mimosa luisanaandCaesalpinia melanadenia, and the succulent nanophanerophyte with shallow roots, E. platyacanthus, flowered and fruited during the rainy season; (2) Phreatophytic shrubs, Prosopis laevigataand Cercidium praecox, and succulent mesophanerophytes with shallow roots, N. tetetzoand C. columna-trajani, flowered and fruited before the rainy season; (3) Castela tortuosa, a shrub with general root system, was the only species to flower and fruit all year long; (4) The semisucculent shrub with general root system, Ipomoea arborescens, flowered and fruited after the rainy season. These phenological patterns were correlated principally with soil moisture, maximum temperature, and to a lesser degree, relative humidity and photoperiod. Our results support the hypothesis that the coexistence of these species in arid zones is due to differences in resource utilization.

Epiphytes experience frequent and rapid changes in water availability and light intensity. The role of carotenoids and tocopherols in photoprotection of seven fern species (Asplenium cuspidatum Lam., Phlebodium areolatum (HB ex Willd.) Smith, Polypodium puberulum Schl. & Cham., Po. plebeium Schl. & Cham., Elaphoglossum glaucum Moode, E. petiolatum (Sw.) Urb., and Pleopeltis mexicana (Fée) Mickel & Beitel) with different adaptations against drought were investigated. The plants were sampled dark adapted (treatment I), and after light exposure with the substrate present (treatment II), with the substrate removed (treatment III) and with substrate and rhizomes removed (treatment IV) to induce increasing degrees of drought stress. The degree of dehydration reached does not result in permanent damage and was also observed in the field. While none of the treatments induced significant chlorophyll (chl) degradation, all plants showed strong de-epoxidation of the xanthophyll cycle on light exposure (up to a de-epoxidation state of 70%), but without an additional effect of desiccation. Most species showed a rapid increase (within hours) of carotenoids (mainly β-carotene and lutein) and α-tocopherol on exposure. In A. cuspidatum, a species with no apparent adaptations to drought, drought stress in combination with light resulted in an increase of tocopherols from 35 nmol mol–1 chl (treatment I) to 400 nmol (treatment IV). This effect was not significant in the drought-deciduous species with succulent rhizomes, Po. puberulum (about 10 nmol mol–1 chl) and Ph. areolatum (5 to maximum 40 nmol), which experience little desiccation under field conditions. This short-term induction of tocopherols and carotenoids has not been reported for other plants under light stress and is probably related to the epiphytic life form. Keywords: antioxidants, Asplenium, carotenoids, drought, Elaphoglossum, epiphytes, ferns, Phlebodium, photooxidative stress, pigments, Pleopeltis, Polypodium, tocopherol, xanthophyll cycle.

We investigated the effects of soil water potential (SWP) and temperature on seed germination of six coexisting species of an inter-tropical desert. These species include three life-forms; the shrubs Cercidium praecoxandProsopis laevigata; the columnar succulents Neobuxbaumia tetetzoand Pachycereus hollianus; and the arborescent semi-succulents Beaucarnea gracilisand Yucca periculosa. In the six species germinability increased and germination time and speed of germination (t50) decreased as SWP decreased. The SWP treatments were 0 MPa, −0·12 MPa, −0·2 MPa, −0·41 MPa and −0·66 MPa. There was, however, a SWP threshold below which germination time and t50increased and germinability decreased. The shrubs had the highest germinability whereas the columnar succulents had the lowest. The shrubs also had shorter germination time andt50than arborescent semi-succulents whereas seeds of the columnar succulents were indeterminate. In all species except P. laevigatagerminability increased and the germination time and t50decreased as temperature increased. The temperature treatments were 12°C, 20°C and 26°C. The shrubs had the shortest t50and germination time and the highest germinability at all temperatures. Arborescent semi-succulents had the lowest germinability and longest germination time and t50at the three temperatures treatments. Our results support the hypothesis that in desert environments different plant life-forms utilise different germination strategies to persist.

In a semidesert community in México (Zapotitlán de las Salinas, Puebla) the vertical distribution of roots and root biomass was estimated at 0–100 cm depth on two sampling dates, November 1995 (wet season) and January 1998 (dry season). Root productivity at 7 to 14.5 cm depth was estimated with the in-growth core technique every two months from March 1996 to February 1998. The relationship between environmental factors and seasonal root productivity was analyzed. Finally, we tested the effect of an irrigation equivalent to 20 mm of rain on root production. Seventy four percent of the total number of roots were found at 0-40 cm depth. Very fine roots (<1 mm diameter) were found throughout the soil profile (0-100 cm). In contrast, fine roots (1-3 mm diameter) were found only from 0–90 cm depth, and coarse roots (>3 mm diameter) from 0–60 cm depth. The root biomass was 971.5 g m−2(S.D. = 557.39), the very fine and fine roots representing 62.9% of the total. Total root productivity, as estimated with the ingrowth core technique, was 0.031 Mg ha−1over the dry season and 0.315 Mg ha−1over the wet season. Only very fine roots were obtained at all sampling dates. Rainfall was significantly correlated with very fine root production. The difference between fine root production in non-watered (0.054 g m−2) and watered (0.429 g m−2) treatments was significant. The last value was the same as that predicted for a rain of 20 mm, according to the exponential model describing the relation between the production of very fine roots and rainfall at the site.

Soil water use by shrubs and grasses of vegetation patches (vegetation arcs) occurring in two-phase mosaics of the Southern Chihuahuan Desert (Mexico) was investigated after an experimental irrigation equivalent to a 75 mm rainfall. Three shrubs (Flourensia cernua,Larrea tridentataandProsopis glandulosa) and one grass (Hilaria mutica) were studied.Irrigation water did not percolate deeper than 40 cm. This soil layer contained more than 75% of the roots in all species, exceptP. glandulosawhere a less developed, deeper root system was detected (but not quantified).Root distribution indicates that the water stored in the 0–40-cm soil layer after the experimental irrigation was available for the four species. However, predawn xylem water potential (XWP) ofF. cernuaandH. muticawere strongly influenced by soil water present in the 0–40-cm layer, whereas those ofL. tridentataandP. glandulosawere not. Differences in predawn XWP between watered and unwatered individuals were greatest inF. cernuaandH. mutica, and smallest inP. glandulosa. Changes in tissue osmotic potential (TOP) values as a consequence of watering were sharply marked in all species exceptP. glandulosa. H. muticaXWP approached zero for a few days in response to small rain events.The results indicate that adult individuals of grasses and shrubs are potential competitors for soil resources (to a variable degree according to the shrub species). Their co-existence in the arcs is probably favoured by a process of slow competitive displacement as long as the recruitment of new shrubs takes place mainly by colonization of the upslope fringe of the arcs where grass biomass is low. As development of the vegetation progresses in the colonization front and the grass canopy is almost closed, the chances of a shrub being suppressed by water competition diminishes in the following order:F. cernua,L. tridentata,P. glandulosa. A drastic reduction in grass biomass because of grazing would depress the competitive ability of the grasses and may preferentially facilitate the establishment of livestock dispersed species likeP. glandulosa. A consequent shift to a more shrubby community seems unavoidable since the recovery of the grass strata will probably not suppress the newly-established shrubs tapping water from deep water sources.