Potential Soil Water Deficit Research Articles

The Rate of Canopy Development Modulates the Link Between the Timing of Spring Leaf Emergence and Summer Moisture

AbstractShifts in phenological timing have important implications for ecosystem processes, with spring leaf emergence as a dominant control of carbon, water, and energy cycling. Phenological events are predominantly determined by weather and climate, therefore dynamic in time and sensitive to climate feedbacks. Improving our understanding of how ecosystems respond to changes in phenological timing will enhance our ability to assess summer soil water availability, since the timing of spring leaf emergence may lead to soil moisture deficits later in the growing season. We leveraged data from five AmeriFlux towers in central and eastern United States to investigate the extent spring leaf emergence (i.e., start of spring, SoS) influences rates at which forest canopies develop and how this impacts summer soil moisture (θJJA) variability. Our results indicate that ecosystem processes, specifically gross primary production (GPP) and evapotranspiration (ET), exhibit compensatory responses to varying leaf emergence; with delayed spring‐onset, the canopy developed more quickly, resulting in rapid GPP and ET increases, consistent across sites. Nonetheless, early SoS is a relatively good indicator for potential summer soil water deficits, particularly when it occurs together with meteorological conditions (i.e., lower‐than‐average precipitation, hot summer temperatures) that contribute to soil water deficits. When these meteorological conditions coincide with early SoS, θJJA deficits are exacerbated. To the extent that these extreme conditions occur more frequently under future climate scenarios, the dynamics of spring phenology and hydroclimate may play an increasingly important role in portending the likelihood of summer water deficits, which are projected to become more severe.

Diversity of soil organic carbon and water characteristics under different vegetation types in northern Bengkulu, Indonesia

Abstract. Hermawan B, Suhartoyo H, Sulistyo B, Murcitro BG, Herman W. 2020. Diversity of soil organic carbon and water characteristics under different vegetation types in northern Bengkulu, Indonesia. Biodiversitas 21: 1793-1799. Organic carbon and water characteristics are important soil properties that can easily degrade following the conversion of forest to cultivation land, but their levels of degradation depend on successive vegetation types after deforestation. The research aimed to evaluate the spatial diversity of soil organic carbon (SOC) and water characteristics on sites with annual and cash crops, as well as natural vegetation types. Forty-two undisturbed and disturbed soil samples were collected at 0-10 and 10-20 cm depths from plantation areas (rubber and oil palm), rice fields, shrub land, and grassland in Mukomuko, North Bengkulu, and Central Bengkulu Regencies of Bengkulu Province, Indonesia. Vegetation types were observed, and soil water characteristics were determined using a pressure plate apparatus unit from undisturbed soil samples, while SOC and particle-size distribution were analyzed from disturbed soil samples. The uppermost 10 cm soil layers were dominated by organic-C intervals of more than 3.0% while the 10-20 cm depths had intervals of less than 3.0%. Soils under oil palm and rice production had higher SOC of above 6.0% and water availability at the field capacity of 0.4-0.6 g g-1, particularly at 0-10 cm depths, while those under rubber and grass had a lower status of measured variables, particularly at 10-20 cm depth. There was a strong relationship (R2 = 0.63) between the soil organic carbon and water availability among study soils, suggesting the possibility to predict potential soil water deficits from organic matter status.

Evaluation of recent drought conditions by standardized precipitation index and potential evapotranspiration over Indonesia

In South Asia, an increasing population and frequent droughts have been significant factors deeply affecting water deficits in the region. In this study, recent drought conditions were evaluated by calculating the standardized precipitation index (SPI) for the period of 1991–2006, based on past data during the period from 1961 to 1990. In addition, the Thornthwaite equation was used to compute monthly potential evapotranspiration for the entire area of Indonesia. The APHRODITE data set was utilized for precipitation and temperature. Monthly rainfall data from April to September for 30 years (1961–1990) were used to obtain the gamma function for the computation of SPI values. Calculated probability of SPI for which values were < − 2 during the period from 1991 to 2006 was used to evaluate recent Indonesian drought conditions. Regarding potential soil water deficits, under the very simple assumption that the root zone is 30 cm, soil porosity is 0.4, and field capacity is 80% of soil porosity, the critical threshold of soil water deficit was set as − 96 mm. Frequency of potential water deficits < − 96 mm was counted during 1991–2006. The results of the SPI in Indonesia indicate that most parts of Indonesia have encountered severe and extreme drought for the period 1991–2006. Based on SPI interpretation, Borneo Island and West Papua are the islands that encountered the most extreme drought during the dry seasons. Borneo Island seems to have encountered extreme drought at the beginning and the middle of the dry seasons (April, May, and July). On the other hand, based on the Thornthwaite interpretation, Java and Bali Islands, and especially in Central Java and East Java, seem to have encountered the greatest soil water deficit at the middle and the end of the dry seasons (May, June, July, and September).

Repeated summer drought delays sugar export from the leaf and impairs phloem transport in mature beech.

Phloem sustains maintenance and growth processes through transport of sugars from source to sink organs. Under low water availability, tree functioning is impaired, i.e., growth/photosynthesis decline and phloem transport may be hindered. In a 3-year throughfall exclusion (TE) experiment on mature European beech (Fagus sylvatica L.) we conducted 13CO2 branch labeling to investigate translocation of recently fixed photoassimilates under experimental drought over 2 years (2015 and 2016). We hypothesized (H1) that mean residence time of photoassimilates in leaves (MRT) increases, whereas (H2) phloem transport velocity (Vphloem) decreases under drought. Transport of carbohydrates in the phloem was assessed via δ13C of CO2 efflux measured at two branch positions following 13CO2 labeling. Pre-dawn water potential (ΨPD) and time-integrated soil water deficit (iSWD) were used to quantify drought stress. The MRT increased by 46% from 32.1 ± 5.4 h in control (CO) to 46.9 ± 12.3 h in TE trees, supporting H1, and positively correlated (P < 0.001) with iSWD. Confirming H2, Vphloem in 2016 decreased by 47% from 20.7 ± 5.8 cm h-1 in CO to 11.0 ± 2.9 cm h-1 in TE trees and positively correlated with ΨPD (P = 0.001). We suggest that the positive correlation between MRT and iSWD is a result of the accumulation of osmolytes maintaining cell turgor in the leaves under longer drought periods. Furthermore, we propose that the positive correlation between Vphloem and ΨPD is due to a lower water uptake of phloem conduits from surrounding tissues under increasing drought leading to a higher phloem sap viscosity and lower Vphloem. The two mechanisms increasing MRT and reducing Vphloem respond differently to low water availability and impair trees' carbon translocation under drought.

Linacre Derived Potential Evapotranspiration Method and Effect on Supplementary Irrigation Water Needs of Tomato/Cabbage/Carrot

The study examined the dynamic nature of water balance parameters over Kano town, a semi-arid environment and impact of Linacre derived potential evapotranspiration method on the supplementary irrigation water needs of selected crops. Monthly Rainfall and Temperature data were collected from the Nigerian Meteorological Agency, Lagos for the period 1953-2012. The study observed that there is a steady decline in annual precipitation over Kano from the first decade (1953-1962) to the fifth decade (1993-2002), after which there was a sign of weak recovery in the last decade (2003-2012). For water loss through potential evapotranspiration, there was a steady rise from the first decade (1953-1962) to the fifth decade (1993-2002), and then followed by a sudden decline in the last decade (2003-2012). The total average of water storage on the other hand, first experienced a rise between the first two decades (1953-1962) and (1963-1972), followed by a steady decline, up until the fifth decade (1993-2002) and finally a rise in the last decade (20032012). The total average of soil water deficit experienced a steady rise between the first and the fifth decades (1953-1962) to (1993-2002), this was followed by a decline in the last decade (20032012). Finally, the total average of water surplus experienced a steady decline between the first and the fifth decades. The observed decline in precipitation, storage, and water surplus, and the rise in water loss from potential evapotranspiration and soil water deficit, suggests that there have been changes in the climatic pattern over Kano and this could be seen in the supplementary irrigation water needs of Tomato/Cabbage/Carrot.

Yield and quality responses of carrots (Daucus carota L.) to water deficits

ABSTRACTThere is little quantitative information available on the responses of carrots (Daucus carota L.) to water deficits. We tested the hypothesis that root yield decreases with increases in the maximum potential soil water deficit (DØmax, mm). Field crops of carrot (Chantenay Red Core) were grown under a rain-out shelter near Lincoln, New Zealand, using trickle irrigation to achieve six treatments that differed in the timing (early-, mid- and late-season) and intensity (moderate and high) of water stress. There were also unirrigated and fully irrigated (minimal water stress) treatments. Total biomass and root yields declined linearly with increasing DØmax, supporting the hypothesis under test. Storage root diameter, but not length, also decreased, so that process-grade packouts of roots fell sharply with DØmax. There was no significant relationship between root soluble solids concentration and DØmax. Minimising water deficits is crucial for achieving high yields of quality carrots.

Physiology of yield determination of soybean ( Glycine max (L.) Merr.) under different irrigation regimes in the sub-humid zone of Sri Lanka

Water stress is a major constraint to increasing soybean yields in the sub-humid zone of Sri Lanka. Therefore, the present study was conducted with the objective of analyzing the yield variation of soybean in terms of leaf area growth, radiation interception, radiation use efficiency (RUE) and harvest index (HI) under a range irrigation regimes at Maha-Illuppallama in the sub-humid zone of Sri Lanka (7–8°N and 80–81°E) in 1995 and 1996. Eight irrigation regimes were defined as all possible combinations of irrigation/non-irrigation during three growth stages, i.e. vegetative (V 1–R 1), flowering (R 1–R 4) and pod-filling (R 4–R 8). Consequently, the maximum potential soil water deficit (PSWD) ranged from 69 mm in the ‘fully irrigated’ treatment to 462 mm in the rainfed treatment. The maximum leaf area index (LAI, L m), the fraction of incoming radiation intercepted ( F i) and the maximum total biomass ( W m) increased significantly with the number of stages irrigated, with irrigation during the vegetative stage having the highest positive effect. Reductions in L m, F i and W m due to the absence of irrigation during the vegetative stage, could not be recovered completely by irrigation during the two subsequent stages. RUE also increased significantly with the number of stages irrigated, with irrigation during the flowering and pod-filling stages showing a greater positive effect. Although irrigation during the vegetative stage promoted leaf area growth and radiation interception, absence of irrigation during the subsequent stages decreased RUE significantly because of the substantial water deficits due to higher water loss from the larger canopies. The maximum number of pods (NP) initiated, the mean pod growth rate (PGR) and HI increased significantly with the number of stages irrigated with irrigation during the flowering stage having the highest positive effect. Mean PGR during the main pod-filling period exceeded the corresponding overall crop growth rate (CGR) in all treatments, indicating re-translocation of assimilates from vegetative organs. The highest re-translocation was observed in the treatment which was irrigated during vegetative and flowering stages, but was kept rainfed during the pod-filling stage.

A COMPARISON OF THE RESPONSES OF MATURE AND YOUNG CLONAL TEA TO DROUGHT

To assist commercial producers with optimizing the use of irrigation water, the responses to drought of mature and young tea (Camellia sinensis) crops (22 and 5 years after field planting respectively) were compared using data from two adjacent long-term irrigation experiments in southern Tanzania. Providing the maximum potential soil water deficit was below about 400–500 mm for mature, and 200–250 mm for young plants (clone 6/8), annual yields of dry tea from rainfed or partially irrigated crops were similar to those from the corresponding well-watered crops. At deficits greater than this, annual yields declined rapidly in young tea (up to 22 kg ha−1 mm−1) but relatively slowly in mature tea (up to 6.5 kg ha−1 mm−1). This apparent insensitivity of the mature crop to drought was principally due to compensation during the rains for yield lost in the dry season. Differences in dry matter distribution and shoot:root ratios contributed to these contrasting responses. Thus, the total above-ground dry mass of well-irrigated, mature plants was about twice that for young plants. Similarly, the total mass of structural roots (&gt;1 mm diameter) to 3 m depth was four times greater in the mature crop than in the young crop and, for fine roots (&lt;1 mm diameter), eight times greater. The corresponding shoot:root ratios (dry mass) were about 1:1 and 2:1 respectively. In addition, each unit area of leaf in the canopy of a mature plant had six times (by weight) more fine roots available to extract and supply water than did a young plant. These results show that young tea should be irrigated in preference to mature tea, especially where the maximum soil water deficit is likely to exceed 250 mm.

Physiology of yield determination of mung bean ( Vigna radiata (L.) Wilczek) under various irrigation regimes in the dry and intermediate zones of Sri Lanka

Drought is a major factor limiting yield improvement of mung bean ( Vigna radiata (L.) Wilczek) in the sub-humid, dry and intermediate zones of Sri Lanka. Therefore, the objective of this study was to analyze the yield response of mung bean to irrigation at various phenological stages in terms of radiation interception, radiation-use efficiency and harvest index. Four field experiments were carried out at two sites (Maha-Illuppallama and Kundasale) during the short, dry yala season over two years (1995 and 1996). The life cycle of mung bean was divided into three stages: vegetative (from germination to appearance of first flower); flowering (from appearance of first flower to 75% pod initiation); and pod-filling (from 75% pod initiation to maturity). Eight irrigation treatments were defined as all possible combinations of irrigation during the three stages. Maximum potential soil water deficits (PSWD) ranging from 127 to 376 mm developed as a result of keeping different combinations of stages unirrigated. Maximum LAI ( L m) and the fraction of incoming radiation intercepted ( F) increased significantly with the number of stages irrigated. Specifically, treatments which included irrigation during the vegetative stage achieved large L m and F. Radiation-use efficiency (RUE), maximum total biomass ( W m), harvest index (HI) and seed yield ( Y) also showed a significant positive response to the number of stages irrigated. However, all the above parameters were significantly greater in treatments which included irrigation during the pod-filling and flowering stages. The treatment which received irrigation only during the vegetative stage had significantly lower RUE, W m, HI and Y despite having higher L m and F. Therefore, irrigation is critical during pod-filling and flowering stages mainly because of the higher LAI during these periods and, consequently, the greater demand for water. Lack of irrigation during these critical stages resulted in the development of significant PSWD with adverse effects on photosynthesis and consequently decreased RUE. Moreover, water stress during flowering and pod-filling stages significantly reduced pod initiation and pod growth rates and thereby reduced HI. It is concluded that to maximize mung bean yields in the dry season of the sub-humid zones of Sri Lanka, irrigation should extend across all phenological stages, specially the pod-filling stage.

THE USE OF LEAF APPEARANCE RATES ESTIMATED FROM MEASUREMENTS OF AIR TEMPERATURE TO DETERMINE HARVEST INTERVALS FOR TEA

The effects of temperature (13–20 °C) and potential soil water deficit (0–350 mm on the leaf appearance rates of six contrasting clones of tea (Camellia sinensis) were studied over twenty-five months to provide guidance to tea growers in southern Tanzania on the choice of harvest intervals. Within the fully irrigated treatments (potential soil water deficit &lt;65 mm) the mean time period between a shoot unfurling its second and third true leaf (a phyllochron) ranged from 5.8–7.9 d during the warm–wet season to 11–19 d during the cool-dry season. Between 61 and 79% of the weekly variation in the mean leaf appearance rate (1/phyllochron) for each clone could be explained by an asymptotic relation with the mean air temperature. Building on research in India which has suggested that appropriate harvest intervals for tea can be derived from measurements of leaf appearance rates, a simple procedure is described for determining harvest intervals from the daily mean air temperature. Corrections to account for the effects of drought and clonal differences in responses to temperature are also explained.

Climate and weather variability at the Tea Research Foundation of Kenya

The climate of the Tea Research Foundation of Kenya (TRFK) is described in terms of year-to-year variability in temperature, humidity, solar radiation, wind, rainfall and evapotranspiration. Thermal time and indices of water stress based on soil water deficits and the saturation deficit of the air are derived. The mean, and 20 and 80% probabilities of exceedance, are calculated for each meteorological variable recorded over 20 years from 1966 to 1986. Mean temperatures vary from 15 to 17°C through the year and the calculated duration of the shoot replacement cycle ranges from 95 to 130 days. Total annual solar radiation (67 TJ ha −1) is relatively constant from year to year. Potentially, there are two dry seasons each year but in 9 years out of 20 the short dry season from September to November does not occur. During the long dry season (from mid-November to the end of March) there is a wide variation in potential soil water deficits which are likely to exceed 350 mm in 4 years out of 10. There are on average 25 hail storms annually of which the majority occur in the period March to October.

Yield variability at the Tea Research Foundation of Kenya

The results of an experiment to monitor the effects of weather variability on the yield of tea ( Camellia sinensis) under constant management over a 16 year period are reported. The overall yield potential was limited to about 2 t ha −1 by the low air and soil temperatures associated with the high altitude (2180 m) which restricted shoot extension rates. The within-year yield distribution was determined by sometimes large potential soil water deficits (up to 400 mm) during the dry season (which restricted yields of made tea by about 1.3 kg ha −1 mm −1) and by the incidence of damaging hail storms throughout the rest of the year (which caused mean losses of 10% of the total annual yield). The implications of this analysis for tea research at Tea Research Foundation of Kenya (TRFK) and elsewhere are discussed.

Responses of Tea (Camellia sinensis) to Irrigation and Fertilizer. I. Yield

SummaryThe yield responses of clonal tea (Clone 6/8) to irrigation and fertilizer were studied in a field experiment (based on the line-source technique) at a high altitude site (1840 m) in the Southern Highlands of Tanzania over a three-year period. In this area (latitude 8°33′S) the annual dry season can last up to six months with potential soil water deficits reaching 600 to 700 mm. In the third year of the experiments yields for the fully irrigated, well fertilized (375 kg N ha−1) treatments had reached 4.9 t ha−1 of made tea. These were reduced by about 2.9 kg ha−1 for each mm increase in the potential soil water deficit. For tea with little or no fertilizer applied the loss of yield was about 1.4 kg ha−1 mm−1. These figures provide a basis for assessing the potential benefits from irrigation where other factors, such as large saturation deficits of the air, do not restrict shoot extension and yield. Yield responses to nitrogen (applied as N:P:K 20:10:10) were essentially linear up to a maximum of about 375 kg N ha−1 in the fully irrigated plots, and 300 kg N ha−1 for the unirrigated plots. Irrigation increased the proportion of crop harvested during the dry season, up to 45% in the fully irrigated treatments. The commercial implications of these results for ‘high’ and ‘low’ input producers are discussed.

Growth and water use of faba beans ( Vicia faba) in a sub-humid climate I. Root and shoot adaptations to drought stress

Root growth, shoot growth and water-use of faba beans ( Vicia faba L.) were studied to ascertain the nature and timing of morphological adaptations to drought stress. Spring-sown crops were grown in an alluvial silt loam in Canterbury, New Zealand, during 1982/83. Extractable water capacity for faba beans in this soil is about 120 mm. The crops were either irrigated frequently to avoid drought stress ( I f treatment) or not irrigated at all ( I o treatment). Grain-yields were 3.8 and 2.9 t ha −1 respectively for the I f and I o crops. Actual soil water-deficit, potential soil water-deficit, soil matric potential at 0.2 m depth, periodic evapotranspiration, and crop height increase were used as indices of drought stress. From around day 70. all the drought-stress indices were greatest in the I o crop. The I o crop responded to stress by reducing its rate of height increase (from about day 70), decreasing its rate of leaf-area expansion slightly (between days 69 and 81), greatly increasing root growth (from about day 78), producing leaves of smaller specific area (from around day 80), and shedding leaves (from around day 94). Radiation conversion efficiency was about 9% greater in the I o treatment. Drought-stress had no detectable effects on shoot dry-matter until after day 120. Thus the observed adaptations were reasonably successful in reducing the drought stress experienced by the I o crop. Furthermore, all these adaptations occurred during flowering and pod-fill, periods suggested by some authors to be when faba beans are especially sensitive to drought. Our results offer some explanation for previous results which suggest faba bean yields are equally sensitive to drought stress during all developmental phases.

The Effects of Prolonged Drought and Nitrogen Fertilizer on Root and Shoot Growth and Water Uptake by Winter Wheat

AbstractWinter wheat growing on a silty clay loam soil was protected from rainfall by a mobile shelter for 100 days from tillering to maturity. During this time the crop was either irrigated according to demand or grew on stored soil water. The effects of this high and low water supply, in combination with a high and low N supply, on root and shoot growth and water uptake were studied.The crop given both N and water yielded 9.7 t/ha of grain (85 % DM), drought reduced this to 7.9 t/ha, low N to 4.3 t/ha and drought and low N to 3.8 t/ha. Yield reductions were mainly due to fewer grains being produced.Little root growth occurred in the topsoil during the drought but there was compensatory growth in the subsoil provided that N fertilizer was given. The droughted crops rooted to 160 cm, about 20 cm deeper than the irrigated crops, but the amount of root in the deep subsoil was very small, less than 0.1 cm/cm3 ai 140–160 cm, compared with 5–9 cm/cm3 in the topsoil.The crop demand for water at any given time was partitioned throughout the root system but atmospheric demand was only met whilst the topsoil was wet. The fertilized, droughted crop extracted all of the potentially available water to a depth of 80 cm and a mean rooting density of 1 cm/cm3 was necessary to achieve this. Uptake from below this depth was limited by root growth.The limiting value of the potential soil water deficit was 170 mm, and weather records showed that this would be exceeded one year in ten, on average. The likelihood of yield reduction due to drought could be reduced on this soil by improving root growth below 80 cm depth, although the chances of achieving this are low as root growth was probably limited by poor soil structure.

A Water Stress Index for Tea (Camellia sinensis)

SUMMARYA simple Stress Time Index (STI) for predicting yield loss in the tea crop due to drought is proposed, based on the daily summation of the difference between the potential soil water deficit and a specified limiting value. Validation of the technique with results from a line-source irrigation experiment with a single clone in the Southern Highlands of Tanzania suggests that there is a linear relationship between STI and relative yield loss during the warm dry season. As a result of changes in the composition of the shoot population at each harvest the apparent critical deficit at which shoot growth is restricted increases from below 20 to 300 mm as the dry season progresses. The rate of yield loss with increasing STI also varies through the dry season for the same reason.

The effect of drought on barley yield and water use in two contrasting years

Results are presented for an experiment in 1979 in which mobile shelters were used to exclude rainfall; responses are compared with those in a hotter and drier year, 1976. The continuous drought treatment achieved a larger maximum deficit in 1979 than in 1976. The relationship between rate of water use as a fraction of the potential evaporation rate and soil water deficit was similar in the two years. Yield reductions due to drought were smaller in 1979, as expected from the smaller evaporation rates and therefore smaller potential soil water deficits. Dry matter production was related to water use, but the relationship differed between years. The difference can be related to the saturation vapour pressure deficit. Components of yield were affected differently in the two years; drought after anthesis decreased yield in 1979 by decreasing grain numbers not grain size.

Soil moisture and growth of contrasting varieties of Lolium, Dactylis and Festuca species

AbstractVarieties of Italian, perennial and Italian × perennial ryegrasses, tall fescue and cocksfoot were used to determine the effects of soil moisture on grass growth. Weather conditions were monitored and herbage accumulation, leaf extension rate, leaf appearance rate and tillering were recorded under natural (control), covered and irrigated treatments.Water deficit reduced crop growth rate in the spring and drought was the major factor influencing crop growth rates in all varieties during the summer. The ryegrasses appeared most sensitive to drought, and particularly poor recovery growth was exhibited by the Italian ryegrass RvP and the hybrid ryegrass Snowdon.Leaf extension rate and leaf appearance rate were both reduced by increasing soil water deficit. Herbage accumulation was increased by irrigation when potential soil water deficits were greater than 100 mm. When water deficits were large, irrigation increased leaf extension more than leaf appearance or tiller number. Increasing moisture deficit had a greater effect upon tiller number than on leaf extension.

Use of water by six grass species. 1. Dry-matter yields and response to irrigation

SUMMARYThe yield and quality of herbage produced by six grasses (perennial ryegrass, cocksfoot, timothy, rough-stalked meadow grass, tall fescue and Italian ryegrass) were examined both without irrigation and under two irrigation regimes. Water was applied according to the potential soil water deficit (potential SWD): the soil was either partially returned to field capacity (FC) after each cut or fully returned to FC whenever the potential SWD reached 25 mm. The swards were cut either at 3 (C3) or 6 (C6) week intervals over a 2 year period.Partial irrigation increased yields by 12–14% in the first year and by 36–58% in the second. Full irrigation produced little more growth than partial irrigation in the first year (maximum SWD, 188 mm) but increased yield by 78–93% in the second, very dry, year (maximum SWD, 311 mm). Under treatment C3 response per unit of water applied was similar with both partial and full irrigation, but under C6 the response was greater with partial (2·86 kg D.M./m3) than with full irrigation (1·79 kg D.M./m3).There were marked differences between the species in their ability to grow under drought conditions in the second year of the experiment. Without irrigation, roughstalked meadow grass and Italian ryegrass did not survive the drought. The performance of tall fescue was markedly superior to both perennial ryegrass and cocksfoot in these conditions. Of the surviving grasses timothy made least growth.

A drought experiment using mobile shelters: the effect of drought on barley yield, water use and nutrient uptake

SummaryAutomatic mobile shelters were used to keep rain off a barley crop in a drought experiment. The treatments ranged from no water during the growing season to regular weekly irrigation. This paper reports the effect of drought on the harvest yield and its components, on water use and nutrient uptake.Drought caused large decreases in yield, and affected each component of the grain yield. The magnitude of each component varied by up to 25% between treatments, and much of the variation could be accounted for by linear regression against the mean soil water deficit in one of three periods. For the number of grains per ear, the relevant period included tillering and ear formation; for the number of ears per unit ground area, the period included stem extension and tiller death; for grain mass, the period included grain filling.The harvest yields were linearly related to water use, with no indication of a critical period of drought sensitivity. The relation of grain yield to the maximum potential soil water deficit did show that a prolonged early drought had an exceptionally large effect on both yield and water use.Two unsheltered irrigation experiments, also on barley, were made in the same year on a nearby site. The effects of drought on yield in these experiments were in good agreement with the effects observed on the mobile shelter site.When fully irrigated, the small plots under the mobile shelters used water 11% faster than larger areas of crop, because of advection. The maximum depth from which water was extracted was unaffected by the drought treatment. When 50% of the available soil water had been used the uptake rate decreased, but the maximum depth of uptake continued to increase.Measurements of crop nutrients at harvest showed that nitrogen uptake was large, because of site history, and that phosphate uptake was decreased by drought to such an extent that phosphate shortage may have limited yield.