Summer Rain Events Research Articles

EVALUATION OF VERTICAL AND LATERAL FLOW THROUGH AGRICULTURAL LOESSIAL HILLSLOPES USING A TWO-DIMENSIONAL COMPUTER SIMULATION MODEL

On four hillslopes in the loess region of The Netherlands pressure heads were monitored using stand alone measuring devices. During rain events pressure heads were measured regularly with time intervals of five minutes. Rainfall was recorded with a tipping bucket rainfall gauge. Two-dimensional simulations were executed to simulate water flow through these hillslopes during erosive rain events in summer and winter periods. From these simulations it appeared that vertical flow is dominant during rain events, and that lateral water transport is of minor importance despite distinct layer stratifications. During the selected rain showers, average lateral water movement varied between 1.6 and 4.7% of the total water displacement through the hillslopes. Therefore, it was decided to incorporate a one-dimensional water flow module into the event-based distributed soil erosion and hydrological model LISEM.

MEASURING AND MODELLING OF SOIL WATER DYNAMICS AND RUNOFF GENERATION IN AN AGRICULTURAL LOESSIAL HILLSLOPE

Surface run-off may be generated when rainfall intensity exceeds infiltration capacity, or when the soil profile is saturated. Both types of overland flow may occur in hilly agricultural loess regions. It was shown with pressure head and run-off measurements that Hortonian overland flow occurs during summer rain events. A two-dimensional water flow model could simulate pressure head changes and run-off. Simulated potential run-off for the transect studied was three times as high. This indicates effects of surface ponding and the probable location of this particular transect in a region with high run-off production.

MEASURING AND MODELLING OF SOIL WATER DYNAMICS AND RUNOFF GENERATION IN AN AGRICULTURAL LOESSIAL HILLSLOPE

Surface runoff may be generated when the rainfall intensity exceeds the infiltration capacity, or when the soil profile is saturated with water. Indications exist that both types of overland flow may occur in hilly agricultural loess regions. Here, for a loessial hillslope under maize in the southern part of The Netherlands, it was shown, with pressure head and runoff measurements, that Hortonian overland flow occurs during typical summer rain events. Surface runoff was initiated after saturation of the top 5–10 cm of the soil. Deeper in the soil, unsaturated conditions prevailed while runoff took place. Peak runoff discharges at the outlet of the subcatchment occurred a few minutes after peak rainfall intensities were measured. It appeared that SWMS_2D, a two-dimensional water flow model, was capable in simulating observed pressure head changes and runoff. Simulated potential runoff for the transect studied was higher by a magnitude of three than the measured areal average. This indicates effects of surface ponding, and the probable location of this particular transect in a region with high runoff production.

Monosoonal precipitation responses of shrubs in a cold desert community on the Colorado Plateau.

South-eastern Utah forms a northern border for the region currently influenced by the Arizona monosoonal system, which feeds moisture and summer precipitation into western North America. One major consequence predicted by global climate change scenarios is an intensification of monosoonal (summer) precipitation in the aridland areas of the western United States. We examined the capacity of dominant perennial shrubs in a Colorado Plateau cold desert ecosystem of southern Utah, United States, to use summer moisture inputs. We simulated increases of 25 and 50 mm summer rain events on Atriplex canescens, Artemisia filifolia, Chrysothamnus nauseosus, Coleogyne ramosissima, and Vanclevea stylosa, in July and September with an isotopically enriched water (enriched in deuterium but not 18O). The uptake of this artificial water source was estimated by analyzing hydrogen and oxygen isotope ratios of stem water. The predawn and midday xylem water potentials and foliar carbon isotope discrimination were measured to estimate changes in water status and water-use efficiency. At. canescens and Ch. nauseosus showed little if any uptake of summer rains in either July or September. The predawn and midday xylem water potentials for control and treatment plants of these two species were not significantly different from each other. For A. filifolia and V. stylosa, up to 50% of xylem water was from the simulated summer rain, but the predawn and midday xylem water potentials were not significantly affected by the additional summer moisture input. In contrast, C. ramosissima showed significant uptake of the simulated summer rain (>50% of xylem water was from the artificial summer rain) and an increase in both predawn and midday water potentials. The percent uptake of simulated summer rain was greater when those rains were applied in September than in July, implying that high soil temperature in midsummer may in some way inhibit water uptake. Foliar carbon isotope discrimination increased significantly in the three shrubs taking up simulated summer rain, but pre-treatment differences in the absolute discrimination values were maintained among species. The ecological implications of our results are discussed in terms of the dynamics of this desert community in response to changes in the frequency and dependability of summer rains that might be associated with a northward shift in the Arizona monsoon boundary.

Limited uptake of summer precipitation by bigtooth maple (Acer grandidentatum Nutt) and Gambel's oak (Quereus gambelii Nutt)

Winter and spring precipitation that saturates to deep soil layers precedes summer droughts in the Intermountain West. Occasional summer convection storms relieve summer drought, but are infrequent and unreliable from year to year, leading to the hypothesis that dominant tree species might not invest limited carbon reserves to surface roots to take up summer precipitation in these regions. We compared the hydrogen (δD) and oxygen (δ18O) isotope ratios of winter, spring and summer precipitation to that of xylem sap water in Acer grandidentatum and Quercus gambelii, two dominant trees of this region. By this method we could identify water sources utilized throughout the growing season. Xylem δD and δ18O values changed significantly when each species leafed-out; this change was not associated with changes in either soil or plant water status (as measured by predawn and midday water potentials). This shift is apparently related increased transpirational flux, which may flush out residual stem water from the previous growing season. δD values of xylem sap of both species matched winter precipitation input values throughout most of the summer, indicating a reliance on deep-soil moisture sources throughout the growing season. Mature Q. gambelii did not take up summer precipitation, whereas A. grandidentatum responded slightly to the largest summer rain event. Small trees of both species, particularly A. grandidentatum, showed a limited uptake of summer rains.

Precipitation Chemistry and Chemical Limnology of Fertilized and Natural Lakes at Saqvaqjuac, N.W.T.

Precipitation at Saqvaqjuac (northwest coast of Hudson Bay, 60°39′N) had high concentrations of sea salts, was moderately acidic, and had less SO4 and total N than lower latitude precipitation, although SO4 sources were distant. Watershed retention was high for H+, but negative for other elements as a consequence of isostatic rebound. Annual element runoff was a function of the timing of melt runoff and summer rain events because of permafrost. Lake retention of Si was higher than P because of the different times of loading, late summer versus spring. Conservative element mass did not change overwinter except in low-elevation Spring Lake, where residual Cl, Na, and K diffused from the sediments. Phosphorus and N were the only elements incorporated into black ice. As a result of freezeout and incomplete meltwater mixing beneath lake ice, element concentrations were 1.6 times higher in lakes than inflows, and major ion turnover times were 1.5–2.0 times higher than water turnover times.