Contrasting tillage effects on stored soil water, infiltration and evapotranspiration fluxes in a dryland rotation at two locations

Soil & Tillage Research 190 (2019) 157–174

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Contrasting tillage effects on stored soil water, infiltration and evapotranspiration fluxes in a dryland rotation at two locations

T

Robert C. Schwartza, , Alan J. Schlegelb, Jourdan M. Bellc, R. Louis Baumhardta, Steven R. Evetta ⁎

a

USDA-ARS, PO Drawer 10, Bushland, TX 79012, USA Kansas Statte Univ. SW Res. Ctr. 1474 State Hwy. 96, Tribune, KS 67879, USA c Texas A&M AgriLife Research and Extension, 6500Amarillo Blvd W Amarillo, TX 79106, USA b

ARTICLE INFO

ABSTRACT

Keyword: Soil hydrology

There are significant uncertainties in partitioning growing season precipitation into the water balance components that determine water available for dryland crop yield across a range of environments and tillage practices. We evaluated profile soil water contents at a high temporal resolution during phases of a dryland wheat (Triticum aestivum L.) –sorghum (Sorghum bicolor L.) -fallow rotation under no tillage (NT)1 and stubble-mulch tillage (ST) management at Bushland, Texas and Tribune, Kansas to assess event-based soil water balance components. Cumulative infiltration and evaporation were estimated based on hourly changes in stored soil water using a water balance approach and a drainage model. Estimated deep drainage comprised a small proportion of the soil water budget averaging five percent of annual precipitation in Bushland. Tillage did not significantly influence cumulative infiltration during summer fallow and growing season periods at Bushland. In contrast, NT at the Tribune location exhibited significantly greater (P = 0.023) cumulative infiltration compared to ST during the wheat fallow period just prior to sorghum planting. At both locations during summer fallow periods, NT was not more efficient in increasing stored soil water over that obtained with ST. Evaporation during summer fallow periods was more a function of the soil water content near the surface than the tillage practice. Effective infiltration expressed as a fraction of precipitation averaged 0.55 and 0.57 under NT and ST, respectively, at Bushland compared with 0.83 and 0.52 under NT and ST, respectively, at Tribune. Observations of effective infiltration elucidate historical results of the incremental sorghum grain yield response of NT over ST that averaged 12 percent at Bushland and 35 percent at Tribune. Partitioning of growing season precipitation to transpiration was insufficient in Bushland to generate yield increases under NT of similar magnitude to that observed in Tribune demonstrating that NT does not perform similarly across all environments.

1. Introduction Crop rotation is an important production practice that can facilitate weed control, interrupt insect and disease cycles, and sustain productivity. Winter wheat and grain sorghum are well adapted to semiarid conditions where plant-available water is the most limiting yield factor. The winter wheat-sorghum-fallow (WSF) rotation is a common dryland rotation used throughout the southern Great Plains that has been shown to improve annualized yields over that of the wheat-fallow and continuous wheat over the long term (Jones and Popham, 1997) thereby improving water productivity. Conventional tillage practices for dryland production of wheat and sorghum, often referred to as stubble-mulch tillage (ST), generally employ a sweep plow one to four

times during each fallow period to control weeds. Chemical weed control is typically used as a pre-emergent for sorghum and, on a limited basis, during summer fallow periods under these reduced tillage practices (Unger, 1994; Jones and Popham, 1997; Baumhardt and Jones, 2002; Schlegel et al., 2018). No tillage (NT) used within the WSF rotation has exhibited varied results across the region in improving grain yield and water productivity in comparison with reduced or conventional tillage. Mean wheat yields have generally not increased under NT compared with ST in long term studies carried out in the Texas High Plains (Unger, 1994; Jones and Popham, 1997; Baumhardt and Jones, 2002; Baumhardt et al., 2017) and in southwestern Kansas (Norwood, 1994). In west central Kansas, wheat yields have been reported to be not affected by

Abbreviations: AWIGF, adaptive waveform interpretation with Gaussian filtering; CT, conventional tillage; DOY, day of year; E, evaporation; EC, electrical conductivity; ET, evapotranspiration; NP, neutron probe; NT, no tillage; RT, reduced tillage; ST, stubble-mulch tillage; TDR, time-domain reflectometry; WSF, wheatsorghum-fallow ⁎ Corresponding author. https://doi.org/10.1016/j.still.2019.02.013 Received 21 September 2018; Received in revised form 11 February 2019; Accepted 14 February 2019 0167-1987/ Published by Elsevier B.V.

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tillage (Schlegel et al., 1999) and up to 12 percent greater (Schlegel et al., 2018) under NT compared with reduced tillage (RT) in a WSF rotation. In contrast, grain sorghum has generally exhibited positive yield responses under NT compared to ST in west central Kansas and the Texas High Plains. In a long-term study in Tribune, Kansas, mean (1991–2015) sorghum yields were 35 percent greater under NT compared with RT (a total of four to five tillage operations during each 3year rotation) and 108 percent greater under NT compared with conventional tillage (CT) (approximately 4 sweep tillage operations during each fallow phase). In contrast, an eight year study in Bushland, TX (Unger, 1994) exhibited no tillage effect on grain sorghum yield for similar tillage treatments (NT, RT with tillage operations as required only during sorghum fallow, and CT with tillage with operations as required during each fallow phase). Other studies in Bushland, TX have shown marginally greater sorghum yields under NT compared to ST with yield advantages ranging from 7 to 17 percent (Jones and Popham, 1997; Baumhardt et al., 2017). The dichotomy of the incremental sorghum yield response of NT compared with ST at the Tribune and Bushland locations suggests a certain advantageous partitioning of available water during the growing season under NT at the Tribune location. The main driver of grain yield response in these dryland cropping systems is available stored soil water at planting and growing season precipitation (Stone and Schlegel, 2006). Differences between NT and ST stored soil water at planting, typically averaging ˜20 mm in the Bushland studies and less in Tribune, cannot explain the discrepancies in the yield response. In all of these studies, seasonal water use was evaluated based on soil water content measurements at planting and harvest, typically using gravimetric samples although occasionally using the neutron probe. Weekly or biweekly measurements of soil water with the neutron probe have been used in other dryland studies (e.g. Moroke et al., 2005) to provide more time relevant information of crop water use. Although these water balance evaluations provide a general water use assessment, weekly and seasonal measurements cannot capture all of the relevant time scales of precipitation, infiltration, and evaporation processes that ultimately determine water productivity. Recently, Schwartz et al. (2008, 2010) and Guber et al. (2011) estimated profile soil water contents at a high temporal resolution using electromagnetic soil water sensors to assess event-based estimates of the soil water budget. Using this method, rainfall could be partitioned into infiltration and runoff based on the calculated change in storage during the precipitation event. Changes in soil water storage minus the estimated drainage during periods without precipitation were attributed to evaporation. The objectives of this study were to apply the event-based water balance estimation method developed by Schwartz et al. (2008, 2010) to evaluate tillage and location effects on the stored soil water, infiltration and evapotranspiration fluxes within a wheat-sorghum-fallow rotation at the Bushland and Tribune locations. Sorghum grain yield from the instrumented plots as well as historical yield data from the same field and adjacent fields were also used in conjunction with eventbased soil water budgets to elucidate tillage effects on sorghum water productivities and the diverging responses at the two study locations.

The experimental field in Bushland was located on a Pullman clay loam (fine, mixed, superactive, thermic Torrertic Paleustoll) with a 0.5% slope. The Ap horizon (0.0–0.15 m) has a clay content of 380 g kg−1 and a soil organic carbon concentration of 9 g kg−1. The Bt horizon (0.15–1.2 m) in this field has a clay content of 507 g kg−1 that is relatively uniform to 0.75 m (Schwartz et al., 2008) and declines thereafter with depth. Below 1.2 m the Bt transitions into a calcic Btk horizon with a clay loam texture and up to 50% calcium carbonates. Saturated hydraulic conductivities of the Pullman soil (steady-state fluxes recorded at 20 h after initiation of water intake using a double ring infiltrometer) are moderately slow (˜2 mm h−1) (Unger and Pringle, 1981). The Pullman clay loam is also subject to developing a surface seal under NT that can further reduce infiltration rates (Schwartz et al., 2010; Baumhardt et al., 2012). Stored soil water associated with the lower limit of sorghum water use in the Pullman clay loam ranged from 394 to 450 mm for a 2-m profile in a study by Tolk and Evett (2012). The field site at Tribune is 1095 m asl and has a mean annual precipitation of 455 mm with 65% occurring during the growing season. Class A pan evaporation at this location averages 1814 mm for the months of April through October. The experimental field in Tribune was located on a Richfield silt loam (fine, smectitic, mesic Aridic Argiustoll) with a 0.5% slope. The Ap horizon (0.0–0.15 m) has a clay content of 240 g kg−1 and the Bt horizon that extends to 0.40 m has a clay content of 340 g kg−1 (Schwartz et al., 2009a). Below 0.4 m clay contents decline with depth within the BCk and C horizons. Steady-state infiltration rates ranged from 10 to 30 mm h−1 on cultivated land with greater flow rates reported for NT and the wheat phase of the WSF rotation (Stone and Schlegel, 2010). Stored soil water associated with the lower limit of water use in the Richland silt loam is ˜320 mm for a 2-m profile (Stone et al., 2011). In Tribune, the WSF rotation was established in 1991 under three tillage intensities: NT, reduced tillage (RT), and conventional tillage (CT). Conventional tillage consisted of four to five tillage operations with a sweep during fallow periods whereas RT used half of the tillage operations of CT in conjunction with herbicides to control weeds during the fallow periods (Schlegel et al., 2018). The two field plots selected for instrumentation in Tribune consisted of the NT and RT tillage treatments both with the sorghum phase of the rotation occurring in 2006. In Bushland, the WSF rotation was established in 1996 under ST and, beginning in fall 2005, half of the field used in this study was converted to NT. Because of an unacceptably low plant population caused by an intense precipitation event during sorghum emergence in 2006, the study in Bushland was delayed a year whereupon sorghum was again planted in 2007. Thereafter, the wheat sorghum fallow rotation sequence continued uninterrupted. The ST treatment at the Bushland location was most similar to the RT treatment in Tribune, albeit with typically greater tillage intensities after wheat harvest in Bushland. For clarity in the presentation, hereafter the RT treatment in Tribune will be referred to as ST. At Bushland and in Tribune, winter wheat (TAM 111, Foundation Seed,1 College Station, TX) was typically sown in September – October at 45–60 kg ha–1 using a hoe opener grain drill (Bushland and Tribune ST) or a single disc drill (Tribune), harvested in July, and fallowed for 10–11 months. An early maturity grain sorghum (DeKalb DK39Y in Bushland and Pioneer 86G08 in Tribune), was seeded in rows spaced 0.76 m apart at approximately 75,000–100,000 plants ha–1 in late May or June and harvested in October or early November. Thereafter, the field was fallowed another 10–11 months to complete the rotation. Weed control during fallow after wheat and fallow after sorghum was accomplished using a

2. Materials and methods 2.1. Description of field locations and crop rotation Field studies were initiated in 2006 under a winter wheat-grain sorghum fallow (WSF) rotation with no tillage (NT) or stubble-mulch tillage (ST) management in Bushland, TX (35° 11′ 18.6″ N 102° 5′ 8.2″) and Tribune, KS (38° 28′ 12.2″, N 101° 46′ 50.8″). The field site at the Bushland location is 1170 m asl with a mean annual precipitation of 475 mm, of which 67 percent occurs during the growing season from May through September. Mean class A pan evaporation at this location is 2600 mm annually, with 58% occurring during the growing season.

1

The use of trade, firm, or corporation names in this article is for the information and convenience of the reader. Such use does not constitute an official endorsement or approval by the United States Department of Agriculture or the Agricultural Research Service of any product or service to the exclusion of others that may be suitable. 158

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combination of contact and soil active herbicides under NT or tillage with a wide V-blade sweeps (0.9 m and 1.5 m in Bushland; 1.5 m in Tribune) at a depth of 0.08 – 0.1 m under ST. Contact herbicides were also used on both NT and ST fields to treat any weed escapes during the fallow periods. Additional details of the herbicides used for fallow, preemergent, and growing season weed control for this rotation at the Bushland and Tribune locations are given in greater detail by Jones and Popham (1997) and Schlegel et al. (2018), respectively. In Bushland, grain and aboveground biomass of sorghum and wheat were hand harvested (2 rows (1.52 m) × 1 m for sorghum and 4 rows (1.02 m) × 1 m for wheat) from each of the locations with TDR instrumentation or neutron tubes (24 plots). Grain samples were dried in an oven at 70 °C and weighed to determine grain yield and aboveground biomass. Grain yield and aboveground biomass sampling at Tribune is described by Schlegel et al. (2018). All grain yields were adjusted to 130 g kg−1 moisture content. Periodically throughout the study, residue cover in the plots was estimated using the line transect method oriented 45 degrees from the crop row. Color images (0.3 × 0.3 m2) were acquired directly above the array of TDR probes in each of the plots on 8 Jul 2008 in Bushland. Thresholding was carried out manually using a graphics program to produce a 2-color image and thereby determine the percent residue cover.

waveform interpretation with Gaussian filtering (AWIGF) described by Schwartz et al. (2014). All parameters were set equivalent to the default values for interpretation of waveforms using the 1502C cable tester (Schwartz et al., 2014). Soil water contents were calculated from measured apparent relative permittivity εa and temperature T (°C) based on a soil specific calibration: 1

( a , T ) = a + b ( a ) 2 + c T (T

20)

(1)

with parameters a, b and c for each soil and horizon (see Table 1) and based on the measured data obtained by Schwartz et al. (2009a). For the 0.15-m depth increment that lies at the interface between the Ap and Bt horizons at both locations, we used the average of the Ap and Bt coefficients to estimate soil water content. The root mean square errors of this calibration equation (Table 1) for each of the horizons were nearly as small as that obtained by Schwartz et al. (2009a) for these soils using the complex permittivity model. Inclusion of a temperature term offsets the soil permittivity response to temperature (Schwartz et al., 2009b) that is able to correct water contents for diurnal temperature oscillations and for long-term time series as the soil is cooling or warming and thereby improve estimated changes in soil water content. Compared with alternative calibrations using bulk EC as an independent variable, Eq. (1) exhibited smaller root mean square errors and reduced diurnal oscillations associated with soil temperature fluctuations. We note however that Eq. (1) intrinsically assumes that changes in bulk EC are driven principally by changes in temperature and soil water content concomitant with the dilution of dissolved solutes as the soil is wetted. At both locations, micrometeorological instrumentation located on the edge of the experimental field sampled ambient air temperature and relative humidity (model HMP45C Temperature and Humidity Probe, Vaisala Inc., Helsinki, Finland), wind velocity (model 014 A wind sensor, MET-ONE Instruments, Inc, Grants Pass, OR), and total global irradiance (in Tribune model LI-200SA pyranometer, Li-Cor Biosciences, Lincoln, NE and in Bushland model CM14 albedometer at 1 m, Kipp and Zonen, Delft, Netherlands) at 0.25-h intervals and 2 m above the surface. Precipitation was measured using a tipping bucket rain gage (TE525 M, Texas Electronics, Dallas, TX). Short grass reference evapotranspiration (ETref) was calculated using the ASCE standardized reference evapotranspiration equations for hourly intervals (Allen et al., 2005).

2.2. Soil water content monitoring and micrometeorological measurements At each location, plots for monitoring near surface soil water content and temperature were established in adjacent strips of NT or ST. In Bushland, three plots were centered in each of the four 8.2 m × 87 m strips. In Tribune, three plots were centered in each of the two 15.2 × 30.4 m strips. In all cases, plots were located a minimum of 2 m from the strip boundaries and in non-wheel trafficked areas. In each of the 12 plots in Bushland and 6 plots in Tribune, 200-mm trifilar time domain reflectometry (TDR) probes and type-T thermocouples were installed horizontally at soil depths of 0.05, 0.1, 0.15, 0.2, and 0.3 m through an exposed face of small (0.25 m × 0.35 m × 0.35 m) excavated pits. Further details of the instrumental setup are presented by Schwartz et al. (2010). Waveforms and long-time amplitudes for bulk electrical conductivity (EC) calculations were acquired at hourly intervals using a metallic cable tester (Tektronix, Inc., Beaverton, OR, model 1502C) and a computer running the TACQ software (Evett, 2000a, 2000b). Soil temperatures were recorded at 5-min intervals. In Bushland, TDR data was collected from the spring of 2007 until the spring of 2011 (Fig. 1a) with the exception of winter periods when soil near the surface froze and during intermittent power failures or multiplexer problems. Because of several difficulties with the instrumentation in Tribune, useful periods of TDR data only included May-June and October 2006, and May-July 2008 (Fig. 1b). Prior to tillage, TDR probes and thermocouples at the 0.05 and 0.1 m depths were excavated and removed. After tillage, probes and thermocouples were reinstalled at the same location. Soil water contents were also monitored using a neutron probe (model 503DR, InstroTek, Inc., Raleigh, NC) at three locations along the center-line of each of the tillage strips. Neutron probe measurements in Bushland were acquired at depths from 0.1 to 2.3 m in 0.2 m increments at weekly intervals during the growing season and at bi-weekly to monthly intervals at other times during the study. A depth control stand (Evett et al., 2003) was used to ensure that the source was centered at the intended measurement depth. The neutron probe was previously calibrated in 2002 using methods described by Evett and Steiner (1995) with separate equations for the A, Bt, and Btk horizons. Neutron probe measurements in Tribune were monitored at depths from 0.305 to 2.44 m in depth increments of 0.305 and only in 2007 and 2008 from April to September. The probe was previously calibrated in situ for the Richfield silt loam using a single equation for the entire profile (Stone et al., 2011). Travel time of TDR waveforms was evaluated using adaptive

2.3. Determination of soil water balance components Stored soil water and fluxes of infiltration, evapotranspiration, and drainage were calculated for each instrumented plot (i.e. six in Bushland and three in Tribune) based on the event-based water balance estimation procedure of Schwartz et al. (2008, 2010) using TDR and NP-determined water contents and associated precipitation records. Briefly, drainage at the lower boundary of the control volume was estimated using Darcy flux and water potential gradients inferred from measured water contents. Infiltration was estimated based on change in storage during precipitation events, and evaporation or evapotranspiration was calculated based on the soil water balance with known change in storage less drainage during periods without rainfall. Hourly water balance calculations were carried out using scripts written in MATLAB (MathWorks, Natick, MA). Water contents were linearly interpolated for time periods with gaps in acquired TDR data including periods ranging from an hour to six days at Bushland in 2007 and 2008 (Fig. 1a) that were associated with multiplexer or power problems. Because precipitation did not occur during these periods, the calculated water balance at the beginning and end of the gaps was not influenced by the absence of data, although the hourly dynamics during these periods cannot be calculated. In Bushland, stored soil water during the fallow periods was estimated for a 0.6-m control volume by the depth integration of linearly 159

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Fig. 1. Timeline of TDR measurements in relation to rotation phase and tillage (ST only) in (a) Bushland, Texas and (b) Tribune, Kansas.

interpolated TDR (0.0 – 0.3) and NP (0.5 and 0.7 m) soil water contents. The 0.6 m control volume was used rather than the 2.0 m since it was unnecessary to include water stored deeper in the profile in the absence of transpiration and because estimated changes were more precise (less influenced by random fluctuations in neutron probe readings) using the

0.6 m control volume. Drainage out of the control volume was calculated using Darcy flux with the gradient based on the measured soil water contents at 0.5 and 0.7 m and the unsaturated hydraulic conductivity of the 0.6-m interpolated water content. Hydraulic properties in the 0.5–0.7 –m layer were calculated using the van Genuchten160

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to vary among precipitation events. For the data evaluated, which consisted of precipitation events ranging from 9 to 66 mm, the only event for which infiltration fluxes were greatly underestimated was the 66 mm event under NT with a predicted infiltration of 39 mm compared with a HYDRUS -predicted infiltration of 52 mm and a maximum change in water content at 0.3 m during the precipitation event of 0.123 m3 m−3. For all other precipitation events, differences in HYDRUS -predicted and the soil water balance approach cumulative infiltration averaged 0.2 mm with a maximum of 1.0 mm obtained when changes in the 0.3 m water content during the event was ˜0.03 m3 m−3. Based on the preceding results, we adjusted cumulative infiltration, evaporation, and drainage flux during the one precipitation event (6 Oct 2006, day of year (DOY) 299) in Tribune where maximum change in water content at 0.3 m exceeded 0.03 m3 m−3. In Tribune, we used the HYDRUS simulation directly to predict cumulative infiltration, drainage, and evaporation with time for this single event. For the five events in Bushland where maximum change in water content at 0.3 m exceeded 0.03 m3 m−3 during the rainfall event, inverse estimation of soil water contents during precipitation using HYDRUS-1D yielded poor fits of the water contents, dynamics, and overall water balance for this fine textured soil despite using a range of hydraulic property models and layering geometries. It was not possible to properly evaluate fluxes for four of the precipitation events that occurred at the beginning of the wheat growing season in the fall of 2008 and in the spring of 2010 during wheat fallow and we did not use the calculated soil water balance components for these periods in analyses of results. For the remaining event in Bushland where maximum change in water content at 0.3 m exceeded 0.03 m3 m−3, the first of a series of short duration precipitation events during the fallow period of 2008 (DOY 226–230), estimated cumulative infiltration was assumed acceptable because change in water content at 0.3 m marginally exceeded the threshold of 0.03 m3 m−3. However, inspection of the change in stored soil water at 0.6–2.0 m measured using the NP on 12 Aug (DOY 225) and 27 Aug (DOY 240) indicated that calculated cumulative drainage at 0.6 m was underestimated during this period which resulted in an overestimation of evaporation under both tillage treatments. For this time period (DOY 225–240) we used the NP-measured changes in soil water content at 0.6–2.0 m to adjust the total drainage out of the upper 0 – 0.6 m control volume and thereby corrected the cumulative evaporation between DOY 225 and 240 as

Table 1 Coefficients used to estimate TDR- soil water contents using Eq. (1) for the soils and horizons considered in this study. Soil

Pullman Ap Pullman Bt Richfield Ap Richfield Bt

Parameter

MSE

a m3 m−3

b m3 m−3

c m3 m−3 T−2

m3 m−3

−0.1144 −0.05674 −0.1308 −0.1149

0.1029 0.08558 0.1056 0.1037

−3.713E-05 −5.534E-05 −2.299E-05 −3.106E-05

0.0181 0.0242 0.0145 0.0189

Mualem model (van Genuchten, 1980) with calibrated parameters developed for the Pullman clay loam at 0.6 m (Schwartz et al., 2008). During the growing season, soil water storage was estimated for a 2.0-m control volume using linearly interpolated TDR (0.0 – 0.3) and NP (0.5–2.1 m) soil water contents. Darcy fluxes out of the control volume were calculated based on a saturated hydraulic conductivity of 20 mm h−1 and water retention data for the caliche horizon determined by Moroke (2002). At the median measured water content for this horizon of 0.28 m3 m-3 ( ± 0.03 m3 m-3) these parameters yield an unsaturated conductivity of ˜1 × 10-9 m s−1 under a unit hydraulic gradient that approximates conductivities for a caliche horizon estimated by Baumhardt and Lascano (1993). In Tribune, soil water storage was estimated for a 0.61-m control volume in 2007 during the fallow period and a 0.35-m control volume in the spring and fall of 2006. A 0.35-m control volume was used in 2006 because of the absence of neutron probe measurements during this year. Hydraulic parameters for the van Genucten-Mualem model were predicted for these boundaries based on soil texture and water retention at 0.33 bar using the Rosetta pedotransfer functions (Schaap et al., 2001). Cumulative infiltration was calculated from the change in soil water storage with time using the procedures of Schwartz et al. (2008). Smoothing of surface boundary fluxes (e.g., Schwartz et al., 2008; Eq. 11) was unnecessary for this evaluation because of the reduced sampling error of water contents processed with the AWIGF waveform algorithm (Schwartz et al., 2014). Increases in soil water storage during and several hours after precipitation events were used to estimate cumulative infiltration with a time lag τ = 0.2 d between infiltration and measurable increases in soil water and with a threshold hourly accumulation of ε = 0.1 and 0.25 mm associated with the detection limit of the rain gages (i.e., a single tip) in Bushland and Tribune, respectively. In absence of precipitation, changes in soil water storage, which are predominately negative, were attributed to evapotranspiration less calculated drainage. A difficulty encountered with the event-based algorithm occurs when the wetting front extends to a greater depth than the deepest TDR probe prior to the end of the precipitation event and thereby causing an underestimation of infiltration (Schwartz et al., 2008). We emphasize that changes in soil water storage cannot be monitored by the NP frequently enough for adequate time resolution of these transient events. This is problematic with large precipitation events and with coarser textured soils where near saturated hydraulic conductivities are large and the wetting front reaches the deepest probe quickly. We evaluated these potential problems using inverse estimation of soil water contents for precipitation events for the Richfield silt loam in 2006 using HYDRUS-1D (Šimůnek et al., 2013). This permitted the estimation of cumulative infiltration and other boundary fluxes given the known initial conditions, 0.25 h precipitation depths, and hourly ETref and minimization of the objective function consisting of squared deviations between measured and calculated soil water contents. The van Genuchten-Mualem hydraulic model with hysteresis and three soil layers were typically required to obtain differences in measured and predicted storage of less than 3 mm throughout the simulation period. Fitting parameters were restricted to six and optimized values were permitted

240

240

E = S0

240

I + S60

60

225

225

200

+

D200 225

(2)

where E is evaporation, I is infiltration, D200 is calculated Darcy flux drainage at 2.0 m, and ΔS is the change in storage for the control volumes at 0 to 0.6 m and 0.6–2.0 m. 2.4. Assessment of evaporation trends and statistical analysis Direct comparison of calculated evaporation is confounded by varying atmospheric evaporative demands at differing time periods and locations. Based on the work of Black et al. (1969) and Boesten and Stroosnijder (1986), cumulative evaporation E under varying atmospheric conditions and in the absence of transpiration can be described as a function of the square root of the cumulative daily reference ET

E=b

(

ET

ref

)

1 2

+a

(3)

where ETref is daily reference evapotranspiration calculated using hourly weather data and a and b are the linear regression coefficients. The square root of cumulative reference ET term originates from the square root of time relationship obtained assuming isothermal flow and a uniform initial water content with depth (Gardner, 1959). Records of daily cumulative evaporation calculated using the event-based water balance approach for periods without precipitation and greater than 161

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equal to six days were tabulated for all the fallow observation periods and fitted to Eq. (3) to evaluate the slope in response to tillage and the initial measured soil water content at 0.05 m. In the absence of spatial autocorrelation prior to implementation of the tillage treatments (Schwartz et al., 2010), the experimental plot layout was considered to be a completely randomized design at both locations. A one-way analysis of variance (two-sample t-test) was used to evaluate the level of significance of tillage effects (SAS Institute, 2016) with respect to mean water contents and stored soil water at each location at a given time. In addition, one-way analysis of variance was used to evaluate significance levels for tillage comparisons for a time series of plot-averaged infiltration depths and evaporation rates within each rotation phase.

Although the TDR water content calibration Eq. (1) that included temperature as an independent variable reduced the amplitude of the oscillations associated with diurnal temperature fluctuations, the effectiveness of the calibration in reducing these oscillations varied throughout the year and was better for the silt loam soil in Tribune (Figs. 2–3) compared with Bushland (Figs. 4–6). In Bushland, mean water contents estimated using TDR measurements integrated to 0.40 m agreed closely with the mean water contents estimated using the neutron probe (NP) at the 0.1 and 0.3-m depth increments (not shown). For example, during the sorghum growing season in 2007 in Bushland, the root mean square error for these sensor comparisons were 0.024 and 0.010 m3 m−3 for NT and ST, respectively. Direct comparison of TDR and NP water contents estimated at a given depth, for example, 0.3 m, is not possible because of the vastly differing sampling volumes of these two sensors (Schwartz et al., 2018). At both locations, soil water contents under NT were significantly greater than ST in the surface 0.10 m (Figs. 2–6). Differences were most pronounced at the 0.05 m depth in Bushland (mean difference = 0.057 m3 m−3 during fallow after sorghum in 2008; Fig. 5) and the 0.1 m depth water content in Tribune (mean difference = 0.102 m3 m−3 during fallow after sorghum in 2007; Fig. 3). At depths greater than or equal to 0.20 m in Bushland, ST soil water contents were significantly greater compared with NT throughout the sorghum fallow period in 2008 (Fig. 5), with mean differences of 0.045 m3 m−3. In Bushland, the significantly greater water contents under NT at 0.05 m and ST at 0.2 m depths persisted to the end of the study in 2010. By contrast, NT in Tribune had greater water contents compared with ST for the 0 to 0.3 m profile throughout the duration of the study.

greater evaporation under NT resulting in nearly the same change in storage by the end of the season (Table 2). Most of these storage changes (> 80%) during this fallow period occurred within 0.6 m from the surface. Nonetheless, NT maintained a greater profile water content to 2.0 m that averaged 61 mm greater than that for ST throughout the fallow period. The Bushland summer fallow period in 2008 after sorghum (Fig. 5) exhibited an increase of 53 ( ± 8 s.d.) and 39 ( ± 1.2 s.d.) mm of soil water storage within the 2 m profile under NT and ST, respectively, with less than half of the changes in the surface 0.6 m. Significant differences between NT and ST (p < 0.05) in soil water stored during the fallow period were observed for the 2.0 m profile but not at the surface 0.6 m. Despite a precipitation amount of 108 mm from DOY 226 to 233, increases in stored soil water evaluated from DOY 225 to 240 amounted to only an additional 34 and 27 mm for NT and ST, respectively, which largely persisted until wheat emergence on day of year 280. During the sorghum growing season in 2007 at the Bushland location (Fig. 4), despite receiving 186 mm of precipitation, changes in stored soil water associated with infiltration averaged less than a third of this depth under both NT and ST. Only three precipitation events during this period exceeded 10 mm (46% of total precipitation) and the remaining events likely wetted only the first few centimeters and were not fully detected by the TDR probes at 0.05 m because of the small ( ± 25 mm) transverse sensing region (Schwartz et al., 2013), and therefore never entered into the soil water balance measurements. Such small precipitation events are considered ineffective in increasing crop yield because of rapid evaporative losses at the surface. Initial and final soil water contents were nearly identical under NT and ST with a change of storage of 140 mm within the 2.0 m profile largely derived from soil water depletion via sorghum ET (Table 3). The rate of water use by sorghum during the growing season declined rapidly with time with very small changes in storage after DOY 260, one month prior to crop termination by a hard freeze (Fig. 4). Mean grain sorghum yields in 2007 were 3.13 and 3.25 Mg ha−1 for NT and ST, respectively. Water use patterns and changes in storage throughout the wheat phase of the rotation during the spring of 2009 at the Bushland location are virtually identical between NT and ST (Fig. 6). Change in storage associated with infiltration averaged 74% under NT and ST. However, in contrast to the sorghum rotation phase, all precipitation events exceeded 10 mm depth. The increase in the slope of ET around DOY 60 (Fig. 6) corresponds to a warming trend with mean daily soil temperatures at 0.05 m beginning to exceed 15 C. Mean winter wheat yields in 2009 were 1.83 and 1.74 Mg ha−1 for NT and ST, respectively. Summaries of stored soil water and components of the water balance of the remaining periods of the rotation phases in Bushland and Tribune are provided in Tables 2 and 3, respectively.

3.2. Stored soil water

3.3. Drainage

Changes in stored soil water were dominated by evapotranspiration and infiltration fluxes in all the evaluated periods for Bushland and Tribune (Tables 2 and 3). Detailed examples of hourly changes in stored soil water and associated infiltration, evapotranspiration, and drainage fluxes for three evaluation periods are presented in Figs. 2–6. Prior to sorghum emergence in 2006 at the Tribune location (Fig. 2), stored soil water within the surface 0.35 m of the profile was 28 mm greater under NT compared with ST. The greater available soil water under NT can explain a portion of the significantly greater sorghum grain yield (1.82 Mg ha−1) compared with ST (0.20 Mg ha−1) in 2006 (Schlegel et al., 2018), however greater in-season effects on water available for crop growth under NT appeared to be dominating the yield response during this year. In Tribune during the summer fallow period in 2007 (Fig. 3), NT and ST lost an average of 55 ( ± 10 s.d.) and 31 ( ± 2 s.d.) mm stored soil water from the 2.0 m profile, respectively. During this period, NT exhibited greater effective infiltration (73 ± 24 mm) compared with ST (36 ± 3.5 mm) but this added storage was lost via

Darcy flux drainage calculated using a water potential gradient based on measured soil water contents and water retention coefficients is presented in Figs. 2–6 and Tables 2 and 3. In 2008 during the fallow period in Bushland, calculated drainage out of the 0 to 0.6 m control volume between DOY 122 and 225 was 5.0 and 1.7 mm for NT and ST, respectively. The plausibility of the calculated drainage fluxes can be evaluated by comparing with and independent estimate obtained by evaluating the long-term change in stored soil water below 0.6 m during a fallow period. During the same period (DOY 122–225), estimated drainage based on change in soil water contents measured by the neutron probe (0.6–2.0 m), assuming negligible drainage below 2.0 m, was 15.3 ( ± 15.6 95% CI) and 7.3 mm ( ± 7.6 95% CI) for NT and ST respectively. Thus, although calculated Darcy flux drainage was underestimated, the values were within the range of measurement uncertainty for the neutron probe measurements. Similar comparisons for the four other fallow periods in Bushland show that, overall, Darcy flux calculated drainage tended to be greater than drainage calculated based

3. Results 3.1. Soil water contents

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Fig. 2. Mean soil water contents and cumulative water fluxes within the 0- to 0.35-m depth control volume in Tribune, Kansas for the wheat fallow period and continuing into the sorghum rotation phase for ten days after emergence on DOY 165. Shaded regions are 95% confidence intervals and error bars are ± 1 standard deviation for water contents at 0.10 m soil depth and soil water storage, respectively. Large confidence intervals relative to Bushland are caused by a smaller number of observations (n = 3 rather than n = 6).

on change in storage in the 0.6–2.0 m depth increment by an average of 3.6 and 2.2 mm month−1 for NT and ST, respectively. A portion of this difference could likely be attributed to drainage at 2.0 m that was

unaccounted for in the soil water balance approximations. During the 2007 fallow period in Tribune between DOY 117 and 204 calculated Darcy flux drainage was 3.1 mm and -0.3 mm for NT and ST, 163

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Fig. 3. Mean soil water contents and cumulative water fluxes within the 0- to 0.6-m depth control volume in Tribune, Kansas for the summer fallow period after sorghum in 2007. Shaded regions are 95% confidence intervals and error bars are ± 1 standard deviation for water contents at 0.10 m soil depth and stored soil water, respectively. Large confidence intervals relative to Bushland are caused by a smaller number of observations (n = 3 rather than n = 6).

respectively. Respective drainage values based on change in soil water storage were -8.1 and 7.6 mm. Overall, the drainage component comprised only a small proportion of the soil water balance except during episodic periods of significant rainfall events. For example, in Bushland

during DOY 226–230 of 2008, precipitation totaling 105 mm generated an estimated drainage (based on change in storage at 0.6–2.0 m) of 22.6 and 10.4 mm under NT and ST, respectively (Fig. 5). In Tribune, the HYDRUS -estimated drainage flux at 0.35 m for a 67-mm precipitation 164

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Fig. 4. Mean soil water contents and cumulative water fluxes within the 0- to 2.0-m depth control volume in Bushland, TX for in 2007 from sorghum emergence to a hard freeze. Shaded regions are 95% confidence intervals and error bars are ± 1 standard deviation for water contents at 0.05 m soil depth and stored soil water, respectively.

event on DOY 299 in 2006 was 20 and 6.8 mm for NT and ST, respectively (Table 2). Estimated annual drainage at 2.0 m in Bushland based on mean daily drainage rates in this study were 20 and 25 mm y−1 under NT and ST, respectively which are of similar magnitude to

long-term drainage rates based on chloride mass balance estimated by Scanlon et al. (2007) (24 mm y−1), and Baumhardt et al. (2017) (14 and 2 mm y−1 for NT and ST, respectively). 165

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Fig. 5. Mean soil water contents and cumulative water fluxes within the 0 to 0.6-m depth control volume in Bushland, Texas for the summer fallow period after sorghum in 2008. Shaded regions are 95% confidence intervals and error bars are ± 1 standard deviation for water contents at 0.05 m soil depth and stored soil water, respectively. Evaporation and drainage are not predicted for the period between DOY 224 and 240 because drainage past 0.6 m depth was estimated using neutron probe (NP)-determined change in stored soil water (at 0.6–2.0 m depth) and then used to correct evaporation during the period between NP measurements.

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Fig. 6. Mean soil water contents and cumulative water fluxes within the 0- to 2.0-m depth control volume in Bushland, TX for in spring 2009 during the winter wheat growing season. Shaded regions are 95% confidence intervals and error bars are ± 1 standard deviation for water contents at 0.05 m soil depth and stored soil water, respectively.

3.4. Infiltration

phases (Fig. 1). During the fallow period after wheat in 2006, infiltration under NT exceeded that of ST for each event and was significantly greater than ST (P = 0.023; Table 4; Fig. 2). For this period, infiltration expressed as a fraction of the selected precipitation events was 0.78 and

In Tribune, a total of 12 precipitation events with cumulative depths ranging from 9.4 to 66.9 mm were analyzed for the three rotation 167

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Table 2 Detail of the stored soil water for the depth at which it was estimated at the beginning (S0) and ending (Sf) of the rotation phase and the corresponding precipitation (P), infiltration (I), evapotranspiration (ET), and drainage (D) in Tribune, Kansas. Also shown is the stored soil water for the other depth increment in 2007. Year

2006 2006 2007 a b

Phase

Fallow after Wheata Fallow after Sorghumb Fallow after Sorghum

Days

P

Depth

S0

Sf

I

ET

D

mm

m

NT ST NT ST NT ST NT ————————————————————————————————————— mm —————————————————————————————————————

ST

NT

ST

33

115.1

0.35

85.2

59.9

113.3

85.7

89.6

50.2

47.8

23.8

13.7

0.6

20

76.2

0.35

70.6

60.5

97.1

90.8

60.3

53.9

12.5

16.4

21.0

7.2

90

77.2

0.61 1.98

180.1 516

167.9 443

135.4 461

126.0 412

72.7

36.3

120.6

77.9

−3.2 7.1

0.3 −11.2

The fallow after wheat phase includes a portion of the sorghum phase extending from planting to 10 days after emergence. Includes water balance determined using a HYDRUS-1D simulation of a 66.9 mm precipitation event.

0.44 for NT and ST, respectively. Wheat yields in 2005 did not significantly differ with tillage (Schlegel et al., 2018) and therefore greater residue retention rather than stubble production led to the greater residue levels under NT (Table 1S) that likely contributed to increased cumulative infiltration compared with ST. During the sorghum growing season and fallow period after sorghum, infiltration under NT exceeded that of ST in all but one case, however the differences were not significant likely because of limited number of observations (Table 4). In Bushland, a total of 51 precipitation events with cumulative depths ranging from 6.9 to 45.3 mm were analyzed for the eight rotation phases (Fig. 1). From the fallow period after sorghum in 2007 and extending through the wheat phase in 2009, cumulative infiltration was not significantly influenced by tillage (P ≥ 0.407; Table 5) with NT and ST exhibiting similar infiltration depths for each of the precipitation events. Throughout this period, residue cover under NT exceeded ST by an average of 21 percent. During periods after wheat harvest in 2009 (Table 5), there was a trend towards greater infiltration under ST. Infiltration expressed as a fraction of precipitation averaged 0.46 and 0.41 for NT and ST, respectively, during the fallow and sorghum growing season in 2007. Thereafter through fallow after wheat in 2010, these fractions averaged 0.77 and 0.83, respectively. Improved infiltration during and after the wheat phase is likely a result of improved near surface hydraulic properties and greater cover afforded by the plant canopy and residue. In addition, extensive cracking noted in early

June prior to wheat harvest in 2009 that persisted till April 2010 likely contributed to the greater cumulative infiltration fractions of precipitation during these rotation phases. During the sorghum growing season in 2010, these fractions reverted back to low values (≤ 0.42), however there were only two precipitation events during this period. The calculated fraction of precipitation that infiltrated for each of the periods (Tables 2 and 3) was usually less than the analysis based on the limited number of precipitation events exceeding a certain threshold depth (Table 4 and 5) because, for smaller precipitation events not included in Tables 4 and 5, increases in soil water may not be fully detected by soil water sensors as a result of the shallow wetting depth. Water very near the surface (< 2.5 mm) is lost quickly to the atmosphere and consequently does not play an important role in the crop water balance. In Tribune, the overall estimated infiltration expressed as a fraction of the total of all precipitation events was 0.83 and 0.52 for NT and ST, respectively, for the three observation periods. In Bushland, these values were 0.55 and 0.57 for NT and ST, respectively, for the eight observation periods. Coefficients of variation for cumulative infiltration among plots in Tribune ranged from 10 to 33% with variability under NT less than ST for fallow after wheat and variability under NT exceeding ST for fallow after sorghum. Examination of infiltration for plot replicates during the spring of 2006 (not shown) demonstrated that cumulative infiltration for individual plots infrequently exceeded precipitation (twice under

Table 3 Detail of the stored soil water for the depth at which it was estimated at the beginning (S0) and ending (Sf) of the rotation phase and the corresponding precipitation (P), infiltration (I), evapotranspiration (ET), and drainage (D) in Bushland, Texas. Also shown is the stored soil water for the other depth increment. Year

2007 2007 2008 2009 2009 2010 2010

Phase

Fallow after Sorghum Sorghum Fallow after Sorghum Wheata Fallow after Wheat Fallow after Wheatb Sorghumc

Days

P

Depth

S0

Sf

mm

m

NT ST NT ST NT ST NT ————————————————————————————————————— mm —————————————————————————————————————

ST

NT

ST

89

138.6

162

186.1

159

286.1

183

178.0

114

177.3

60

86.9

69

30.9

0.6 2.0 0.6 2.0 0.6 2.0 0.6 2.0 0.6 2.0 0.6 2.0 0.6 2.0

215 650 189 612 164 520 153 564 158 509 198 642 155 578

214 642 189 610 162 514 156 565 168 512 198 631 141 552

189 612 122 474 176 573 159 509 171 528 192 623 122 481

I

189 610 121 468 178 553 169 512 165 516 196 625 115 467

ET

D

53.6

47.2

57.7

55.8

21.8 9.3

16.7 13.5

71.0 173.2

65.2 168.5

200.7 127.5

191.3 137.4

8.1 33.3 3.4

15.5 15.2 9.6

126.5 109.9

128.0 117.6

172.9 95.4

171.3 119.4

60.3

79.6

46.5

66.8

9.0 2.0 2.5 19.3 10.1

9.9 0.8 3.2 15.3 7.2

7.3

11.0

98.8

91.1

5.8

4.3

a For the wheat growing season, cumulative fluxes do not include analysis in (1) the autumn of 2008 because of two large infiltration events totaling 91.8 mm that could not be reliably estimated and (2) the period from 16 Dec 2008 to 7 Jan 2009 because of freezing soil. b Does not include two rainfall events totaling 68.3 mm occurring earlier in the year where infiltration could not be reliably estimated. c Incomplete growing season.

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Table 4 Cumulative precipitation by rotation phase for events exceeding 7 mm and the calculated cumulative infiltration and estimated initial soil water content at 0.05 m depth for no tillage (ST) and stubble-mulch tillage (ST) management in Tribune, Kansas. Probabilities represent a 2-sample t-test comparing infiltration rates between tillage treatments with n events occurring within the rotation phase. Year

Phase

Cumul. Precip.

Mean Event

mm 2006 2007 2008 a b

Fallow after Wheata Fallow after Sorghumb Fallow after Sorghum

105.9 76.3 61.2

17.7 38.2 15.3

Cumulative Infiltration

Mean Initial Soil Water

NT

ST

NT m3 m−3

ST

83.1 60.1 63.3

46.4 53.5 31.9

0.165 0.152 0.157

0.106 0.131 0.109

n

P(T ≤ t)

6 2 4

0.023 – 0.165

The fallow after wheat phase includes a portion of the sorghum phase extending from planting to 10 days after emergence. Includes infiltration estimated using a HYDRUS-1D simulation for a 66.9 mm precipitation event.

NT and never under ST). Mean cumulative infiltration as a fraction of precipitation for this time period ranged from 0.71 to 0.90 under NT and 0.30 to 0.55 for ST. Coefficients of variation for cumulative infiltration among plots in Bushland ranged from 8 to 35% with variability under NT usually exceeding that of ST. Closer examination of the six precipitation events in 2008 from DOY 225 to 231 showed that cumulative infiltration as a fraction of precipitation for plot replicates ranged from 0.45 to 1.29 under NT and 0.41 to 1.71 for ST suggesting localized runoff and runon processes were prevalent during this time period. Under ST for this time period, plot #2 exhibited the greatest infiltration for all six precipitation events. Similarly, under NT, plot #7 exhibited the greatest infiltration for five out of the six precipitation events. Cumulative infiltration for these six precipitation events was poorly correlated to initial soil water contents under both NT (r = -0.13) and ST (r = 0.37). Based on the analyses of RGB images acquired directly above each of the plots on 8 Jul 2008 (DOY 190) (Fig. 6), cumulative infiltration was strongly related to residue cover under NT (r = 0.82) but weakly correlated to residue cover under ST management (r = 0.38). Consistent with the observations of Jones et al. (1994), soil sealing in the absence of residue cover under NT (Fig. 7) likely contributed to the spatial variability of infiltration and reduced overall infiltration depths. The observed variability and temporal persistence of spatial patterns of infiltrations depths at Bushland may also be related to the microlows and microhighs that occur frequently throughout these fields.

exceeded ST except for a brief fallow period in the fall after sorghum harvest (Table 2). During fallow periods, estimated cumulative evaporation was linear with the square root of cumulative ETref with R2 typically exceeding 0.95 and obvious deviations from linear when transpiration was active (Fig. 8). In general, these analyses cannot assess the energy-limiting evaporation stage because it typically ends by the time the drying front reaches the depth of the region sensed by the uppermost TDR probe. Consequently, these calculated evaporation rates pertain to the soil limiting or falling rate stage. At the Tribune location, mean evaporation rate calculated for these short (6–17 day) periods during fallow under NT (1.78 mm d−1) was significantly greater (p = 0.014) than that for ST (0.88 mm d−1). However in Bushland, evaporation rates were not influenced by tillage with NT evaporation rates of similar magnitude (0.97 mm d−1) as for ST (1.00 mm d−1). These tabulated evaporation rates did not consider losses on the day of tillage, which tended to be greater in the spring concomitant with greater near surface soil water contents (Figs. 3 and 5). Rates of evaporation under ST declined significantly after tillage (Fig. 9). These tillage effects can be partly explained by magnitude of the near surface soil water content. As soil water at the 0.05 m depth increased, there was an exponential increase in the scaled slope b2 ETref 1 (Fig. 10). With NT usually exhibiting greater near surface soil water contents at Tribune, it also was usually subject to greater rates of evaporation. Some of the variance in the relationship is likely a result of varying residue cover with time during the study. Greater scaled slopes at lower water contents for the Tribune location are likely attributable to a smaller (less negative) water potential at a given soil water content resulting in less adsorbed soil water at a given relative humidity (Schwartz et al., 2009b; Table 1) and greater unsaturated hydraulic conductivity associated with the textural properties of the Richland soil compared with that for the Pullman clay loam.

3.5. Evaporation Cumulative evaporation during the fallow periods under NT in Bushland was of similar magnitude as ST except during fallow after wheat when ST had greater evaporative losses concomitant with greater infiltration (Table 3). In Tribune, evaporation under NT usually

Table 5 Cumulative precipitation by rotation phase for events exceeding 7 mm and the calculated cumulative infiltration and estimated initial soil water content at 0.05 m depth for no tillage (ST) and stubble-mulch tillage (ST) management in Bushland, Texas. Probabilities represent a 2-sample t-test comparing infiltration rates between tillage treatments with n events occurring within the rotation phase. Year

Phase

Cumul. Precip.

Mean Event

mm 2007 2007 2008 2008-2009 2009 2010 2010 a b

Sorghum Fallow Sorghum Sorghum Fallow Wheata Wheat Fallow Wheat Fallowb Sorghum

98.3 140.5 201.7 206.1 106.9 128.6 22.1

12.3 14.05 15.5 25.8 17.8 32.2 11.05

Cumulative Infiltration

Mean Initial Soil Water

NT

ST

NT m3 m−3

ST

42.4 56.4 156.6 150.5 82.1 107.6 4.6

38.2 56.1 151.7 152.8 94.9 131.8 9.2

0.212 0.123 0.196 0.187 0.156 0.176 0.076

0.155 0.080 0.142 0.156 0.138 0.121 0.045

Possible underestimation of infiltration for two precipitation events in 2008 totaling 91.8 mm. Possible underestimation of infiltration for two precipitation events in 2010 totaling 68.3 mm. 169

n

P(T ≤ t)

8 10 13 8 6 4 2

0.407 0.992 0.889 0.968 0.674 0.126 –

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Fig. 7. Images of residue cover in selected plots in Bushland, Texas on 8 Jul 2008 (DOY 190) and estimated cumulative infiltration as a function of residue cover during the precipitation events occurring from DOY 226 to 230 in 2008 (Fig. 5). Residue cover fractions are shown in parenthesis under each image.

4. Discussion

year, and location (soil) that ultimately determine the crop water use and yield response under differing tillage practices. Long term sorghum grain yield results on the level terraces in Bushland (Jones and Popham, 1997; Baumhardt and Jones, 2002; this study) and the Tribune field

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Fig. 8. Relationship between the square root of reference evapotranspiration and estimated cumulative evaporation Eq. (3) for periods without precipitation under no tillage (NT) and stubble-mulch tillage (ST) in Bushland, Texas (DOY 213–222; 1 Aug – 10 Aug) and Tribune, Kansas (DOY 166–192; 15 Jun – 11 Jul). The deviation from a linear relationship in Tribune occurs in response to weed growth during this period and an herbicide application several days later (Fig. 3).

plots (Schlegel et al., 2018) exemplify the large range in variability in the yield response at a given level of available water (Fig. 11). Available water plotted on the abscissa in Fig. 11 includes all growing season precipitation which is the usual convention for presenting dryland yield response. For the Tribune data set, the two years that exhibited no yield for all treatments were removed in this evaluation to permit a more direct comparison with the Bushland data which did not have any years with zero yield. In Bushland, mean grain yields under NT and ST were 3.69 and 3.30 Mg ha−1, respectively, with 6 of 17 years exhibiting significantly greater yields under NT, but overall, no difference in mean grain yield (P = 0.345). In Tribune, mean grain yields under NT (4.71 Mg ha−1) were significantly (P = 0.022) greater compared with ST (3.33 Mg ha−1), with 9 of the 23 years having harvestable yields exhibiting significantly greater yields under NT. With mean available soil water that ranged from 366 to 386 mm for the two locations, calculated grain water productivity values were 1.21 and 0.88 kg m-3 for NT and ST, respectively, in Tribune and 0.96 and 0.88 kg m-3 for NT and ST, respectively, in Bushland. Normally water productivity of sorghum peaks at around 1.8 kg m-3 (Tolk and Howell, 2008; Bell et al., 2018). Clearly, the larger water productivity for NT in Tribune as well as an incremental yield response obtained for NT with respect to ST at this location that is three times greater than in Bushland, suggests that location is influencing how water is being partitioned into productive (transpiration) and nonproductive (runoff, evaporation, and drainage) fluxes. Analysis of dryland yield response to water customarily distinguishes between stored soil water at planting and in season precipitation (Stone and Schlegel, 2006). Both sources of water have been shown to significantly influence dryland sorghum yields. Differences in initial stored profile water to 2.0 m under NT and ST at sorghum planting were small (≤ 2 mm) in 2007 and 2010 at the Bushland location (Table 3). In 2010, however, a 10 mm surplus of stored soil water to 2.0 m under NT

Fig. 10. Relationship between the volumetric water content estimated using TDR at 0.05 m and the scaled slope b2 ETref 1 of Eq. (3) for the fitted evaporation data in Bushland, Texas and Tribune, Kansas.

compared with ST a day after tillage on day 130 was reduced to a –2 mm deficit prior to sorghum planting on DOY 180 largely as result of greater infiltration under ST and associated filling of the profile below 0.6 m. In Tribune, 29 mm additional water was stored in the 0.35 m profile under NT compared with ST at the time of sorghum emergence. In this study, evaporation rates during the fallow period were more a function of the near surface soil water content regardless of the tillage system. Lasting increases in stored soil water during the fallow were dependent on the precipitation depth, with redistribution into deeper soil depths > 0.3 m occurring only for rainfall depths approximately greater than 25 mm or a series of closely spaced events (e.g. Fig. 5; DOY Fig. 9. Relationship between the square root of reference evapotranspiration and estimated cumulative evaporation Eq. (3) for periods without precipitation under no tillage (NT) and stubble-mulch tillage (ST) in Bushland, Texas (DOY 113–133; 23 Apr – 13 May) and Tribune, Kansas (DOY 118–126; 28 Apr – 6 May). The influence of a tillage event is also shown for ST.

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Fig. 11. Sorghum grain yield response to growing season water use (precipitation + change in stored soil water) for the experimental studies in Bushland, Texas and Tribune, Kansas. All grain yields have been adjusted to 130 g kg−1 moisture content. All Bushland studies are on level terraces. The ST treatment corresponds to the reduced sweep tillage (RT) in Schlegel et al. (2018).

226–230). In both Tribune and Bushland, elevated evaporation rates usually negated any gains in stored soil water afforded by shallower precipitation events thereby reducing differences in stored soil water at planting. Long term studies at Bushland have shown that stored soil water at sorghum planting is often significantly greater under NT compared with ST with mean NT surpluses of 19 (Jones and Popham, 1997) and 28 mm (Baumhardt and Jones, 2002). In contrast, a 25-year study in Tribune (Schlegel et al., 2018) demonstrated no differences in mean stored soil water at planting for NT and RT (reduced sweep tillage). Conventional sweep tillage at this location usually resulted in slightly less stored soil water at sorghum and wheat planting. In Bushland (Moroke et al., 2011; Jones and Popham, 1997; Baumhardt and Jones, 2002) and Tribune (Schlegel et al., 2018), soil water depletion at sorghum harvest was greater under ST thereby partly offsetting the surplus under NT at the beginning of the growing season at the Bushland location and leading to similar seasonal water use under NT and ST assuming runoff is similar. Based on these considerations and an assumed grain water productivity of 1.3 kg m−3, stored soil water at planting can account for approximately 50% of the mean incremental grain yield response of NT compared with ST in the Bushland studies and largely was not important in the Tribune study. Evaluation of yield response to growing season precipitation inherently assumes that the partitioning of yield-forming (transpiration) and non yield-forming (runoff, evaporation, and drainage) water fluxes is static across years and location. In Bushland, estimated drainage below the root zone was approximately 5% (< 16 mm) of growing season precipitation and similar in magnitude to evaluations in other studies (Moroke et al., 2005; Baumhardt et al., 2017) and therefore could be considered negligible. Based on the limited results in this study, drainage could also be considered negligible at the Tribune location. Effective infiltration expressed as a fraction of precipitation averaged 0.83 and 0.52 for NT and ST, respectively, in Tribune for the three observation periods. These results parallel ponded steady-state infiltration rates that were double that under NT compared to ST for the Richfield soil at this location (Stone and Schlegel, 2010). The lack of infiltration evaluations in Tribune during rotation phases prior to, during, and immediately after the wheat growing season signify that the infiltration fractions are only representative of the sorghum and sorghum fallow rotation phases. In Bushland, effective infiltration as a fraction of precipitation were 0.55 and 0.57 for NT and ST, respectively, and unaffected by tillage. In this study, fractions less than unity may result from runoff, canopy interception, or from water that infiltrated but that remained at a shallow depth (< 25 mm) and thus not detected by the TDR sensor. Although Baumhardt et al. (2017) measured runoff in graded terraces during which the sorghum rotation phase was

unaffected by tillage, runoff averaged 5% of growing season precipitation which is far less than runoff observed in this study. However, this neglects decreased runoff normally observed with increasing scale (e.g. Moreno‐de las Heras et al., 2010) and consequently runoff measured with flumes at the end of a 600 m waterway cannot be directly compared with the small plot (˜0.04 m2) observations in this study. In addition, infiltrated precipitation that remains very near the surface (< 25 mm) which, based on our results, can account for approximately 3–6 mm for each precipitation event, is subject to rapid evaporative losses and normally do not contribute to yield (Allen, 2011). Inaccurate runoff approximations, scale effects, and the lack of discrimination between growing season precipitation and effective precipitation in past studies are likely responsible for inconsistent experimental and predicted sorghum grain yield responses to water. For example, Moberly et al. (2017) modeled a sorghum yield response using the Kansas Water Budget (Moberly et al., 2017) that was approximately double that of measured yields. Assuming a growing season precipitation of 275 mm, the fraction of effective precipitation (after runoff) using the Kansas Water Budget relationship was a 0.88 and 0.82 for Tribune and Bushland, respectively. These fractions are considerably larger than the fractions calculated herein and do not consider the influence of tillage on effective precipitation, which for the Tribune location was considerable. The foregoing discussion indirectly implies that the variability of yield response to water in Fig. 11 is partly a result of the variations in effective precipitation across year, tillage, and location. Because the slopes of the yield response to water use for NT and ST in Tribune were not significantly different (P = 0.867), the abscissa representing water use for ST can be shifted to the left to account for reduced available water compared to NT and resulting in a single linear regression for both tillage treatments. The water use shift ΔWU for ST can be calculated as

WU =

b0RT b0ST b1RT

(4)

where b0 and b1 are the intercept and the slope of the linear yield response for each respective tillage treatment (Fig. 11). This results in ΔWU = -73 mm that represents a reduction in mean available water under ST compared with NT. Differences in water available to the crop can also be estimated based on the effective infiltration fractions obtained in this study and a mean precipitation of 269 mm for the yield data in Tribune (Fig. 11) which yields a similar ΔWU value of 269∙(0.52 – 0.83) = –83 mm. Accordingly, a greater effective precipitation fraction under NT as a result of greater infiltration explains the greater water productivity compared with ST in Tribune. Mean wheat yields under NT and ST in Tribune (Schlegel et al., 172

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2018) are 23% greater than those obtained in Bushland (this study; Baumhardt and Jones, 2002; Jones and Popham, 1997). As such, greater residue production and reduced decomposition rates commensurate with lower mean annual temperatures in Tribune likely contribute to, on average, greater residue cover at sorghum planting under NT compared with Bushland. Besides improved infiltration under NT in Tribune, greater residue cover and reduced evaporation during the growing season may contribute to the greater yield response of NT in Tribune compared with Bushland. In Bushland, the marginally greater yield response under NT compared with ST is consistent with similar calculated effective precipitation in this study in conjunction with additional stored soil water at planting and some reductions in growing season evaporation.

throughout the duration of this study. This research was supported by the Ogallala Aquifer Program, a consortium between USDA-ARS, Kansas State University, Texas A&M AgriLife Research, Texas A&M AgriLife Extension Service, Texas Tech University, and West Texas A&M University. Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.still.2019.02.013. References Allen, R.G., 2011. Skin layer evaporation to account for small precipitation events – an enhancement to the FAO-56 evaporation model. Agric. Water Manage. 99, 8–18. Allen, R.G., Walter, I.A., Elliot, R.L., Howell, T.A., 2005. The ASCE Standardized Reference Evapotranspiration Equation. ASCE Press, Reston, VA. Baumhardt, R.L., Jones, O.R., 2002. Residue management and tillage effects on soil-water storage and grain yield of dryland wheat and sorghum for a clay loam in Texas. Soil Tillage Res. 68, 71–82. Baumhardt, R.L., Lascano, R.J., 1993. Physical and hydraulic properties of a calcic horizon. Soil Sci. 155, 368–375. Baumhardt, R.L., Johnson, G.L., Schwartz, R.C., 2012. Residue and long-term tillage and crop rotation effects on simulated rain infiltration and sediment transport. Soil Sci. Soc. Am. J. 76, 1370–1378. Baumhardt, R.L., Schwartz, R.C., Jones, O.R., Scanlon, B.R., Reedy, R.C., Marek, G.W., 2017. Long-term conventional and no-tillage effects on field hydrology and yields of a dryland crop rotation. Soil Sci. Soc. Am. J. 81, 200–209. Bell, J.M., Schwartz, R.C., McInnes, K.J., Howell, T.A., Morgan, C.L.S., 2018. Deficit irrigation effects on yield and yield components of grain sorghum. Agric. Water Manage. 303, 289–296. Black, T.A., Gardener, W.R., Thurtell, G.W., 1969. The prediction of evaporation, drainage, and soil water storage for a bare soil. Soil Sci. Soc. Am. Proc. 33, 655–660. Boesten, J.J.T.I., Stroosnijder, L., 1986. Simple model for daily evaporation from fallow tilled soil under spring conditions in a temperate climate. Netherlands J. Agric. Sci. 34, 75–90. Evett, S.R., 2000a. The TACQ program for automatic time domain reflectometry measurements: I. Design and operating characteristics. Trans. ASAE 43, 1939–1946. Evett, S.R., 2000b. The TACQ program for automatic time domain reflectometry measurements: II. Waveform interpretation methods. Trans. ASAE 43, 1947–1956. Evett, S.R., Steiner, J.L., 1995. Precision of neutron scattering and capacitance type soil water content gages from field calibration. Soil Sci. Soc. Am. J. 59, 961–968. Evett, S.R., Tolk, J.A., Howell, T.A., 2003. A depth control stand for improved accuracy with the neutron probe. Vadose Zone J. 2, 642–649. Gardner, W.R., 1959. Solutions of the flow equation for the drying of soils and other porous media. Soil Sci. Soc. Am. Proc. 30, 425–428. Guber, A., Gish, T., Pachepsky, Y., McKee, L., Nicholson, T., Cady, R., 2011. Event-based estimation of water budget components using a network of multi-sensor capacitance probes. Hydrol. Sci. J. Des Sci. Hydrol. 56, 1227–1241. Jones, O.R., Popham, T.W., 1997. Cropping and tillage systems for dryland grain production in the southern High Plains. Agron. J. 89, 222–232. Jones, O.R., Hauser, V.L., Popham, T.W., 1994. No-tillage effects on infiltration, runoff, and water conservation on dryland. Trans. ASAE 37, 437–479. Moberly, J.T., Aiken, R.M., Lin, X., Schlegel, A.J., Baumhardt, R.L., Schwartz, R.C., 2017. Crop water production functions of grain sorghum and winter wheat in Kansas and Texas. J. Contemp. Water Resour. Res. Educ. 162, 42–60. Moreno‐de las Heras, M., Nicolau, J.M., Merino‐Martín, L., Wilcox, B.P., 2010. Plot‐scale effects on runoff and erosion along a slope degradation gradient. Water Resour. Res. 46, W04503. https://doi.org/10.1029/2009WR007875. Moroke, T.S., 2002. Root Distribution, Water Extraction Patterns, and Crop Water Use Efficiency of Selected Dryland Crops Under Differing Tillage Systems. Ph.D. Diss.. Texas A&M Univer., College Station, TX. Moroke, T.S., Schwartz, R.C., Brown, K.W., Juo, A.S.R., 2005. Soil water depletion and root distribution of three dryland crops. Soil Sci. Soc. Am. J. 69, 197–205 2005. Moroke, T.S., Schwartz, R.C., Brown, K.W., Juo, A.S.R., 2011. Water use efficiency of dryland cowpea, sorghum and sunflower under reduced tillage. Soil Till. Res. 112 (1), 76–84 2011. Norwood, C., 1994. Profile water distribution and grain yield as affected by cropping system and tillage. Agron. J. 86, 558–563. SAS Institute, 2016. SAS/STAT Online Documentation. Version 9.4. SAS Inst., Cary, NC. Scanlon, B.R., Reedy, R.C., Tachovsky, J.A., 2007. Semiarid unsaturated zone chloride profiles: Archives of past land use change impacts on water resources in the southern High Plains, United States. Water Resour. 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5. Summary and conclusions Profile soil water contents were evaluated at a high temporal resolution during phases of a wheat-sorghum-fallow rotation under NT and ST management at two locations to assess event-based estimates of the soil water balance components. Changes in stored soil water were dominated by evapotranspiration and infiltration fluxes in all the evaluated periods at the Bushland and Tribune locations. Overall, the drainage component comprised only a small proportion of the soil water balance except during episodic periods of significant rainfall that increased profile water contents to near field capacity. There were no significant differences in infiltration accumulated under NT and ST during summer fallow and growing season periods at Bushland. In contrast, NT at the Tribune location exhibited significantly greater cumulative infiltration amounts compared to ST during the wheat fallow period just prior to sorghum planting. Considerable spatial variability of event-based infiltration depths suggested that localized runoff and runon transfers occurred, especially at the Bushland location where under NT, cumulative infiltration was strongly correlated to residue cover likely as a result of soil sealing. At both locations, NT was generally no more efficient in increasing stored soil water over that obtained with ST during summer fallow periods. Evaporation at both locations during summer fallow periods was more a function of the soil water content near the surface than the tillage practice. A significant tillage effect on mean grain sorghum yield was not observed in Bushland for this short-term study which parallels historical yield results in the same field and an adjacent level terrace where mean incremental yield increase under NT was 12 percent compared with ST and usually not significant. In Tribune, significantly greater sorghum yields were observed in 2006 under NT compared with ST. Long-term yield results for the same experimental field in Tribune show a significant mean incremental yield response 35 percent greater under NT compared with ST. Based on the observations of this study, the greater yield response under NT in Tribune is a consequence of 60 percent greater mean cumulative infiltration compared with ST. Not all of this additional water is transpired by the crop because the greater soil water contents facilitated greater rates of evaporation. In Bushland, the mean fractions of precipitation that caused detectable changes in stored soil water were similar for NT (0.55) and ST (0.57) signifying that yield improvements would need to come from greater stored soil water at planting and reduced evaporation during the growing season. These two processes were clearly insufficient in Bushland to generate yield increases under NT of similar magnitude to that observed in Tribune. These results suggest that the short-term hydrologic response differs for the soils at these two locations and demonstrates that NT does not perform similarly across all environments. Acknowledgements The authors gratefully acknowledge the assistance of Jeff Slater in Tribune, KS and Grant Johnson, Mandy McCarthy, Collin Acciaioli, and Gretchen Adams in Bushland for field support and data compilation 173

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1D Software Package for Simulating the One-Dimensional Movement of Water, Heat, and Multiple Solutes in Variably-Saturated Media. Dept. Environ. Sc., Univ. Calif. Riverside, Riverside, CA, pp. 308. Stone, L.R., Schlegel, A.J., 2006. Yield-water supply relationships of grain sorghum and winter wheat. Agron. J. 98, 1359–1366. Stone, L.R., Schlegel, A.J., 2010. Tillage and crop rotation phase effects on soil physical properties in the west-central Great Plains. Agron. J. 102, 483–491. Stone, L.R., Klocke, N.L., Schlegel, A.J., Lamm, F.R., Tomsicek, D.J., 2011. Equations for drainage component of the field water balance. Appl. Eng. Agric. 27, 345–350. Tolk, J.A., Evett, S.R., 2012. Lower limits of crop water use in three soil textural classes. Soil Sci. Soc. Am. J. 76, 607–616. Tolk, J.A., Howell, T.A., 2008. Field water supply:yield relationships of grain sorghum grown in three USA southern Great Plains soils. Agric. Water Manage. 95, 1303–1313. Unger, P.W., 1994. Tillage effects on dryland wheat and sorghum production in the southern Great Plains. Agron. J. 86, 310–314. Unger, P.W., Pringle, F.B., 1981. Pullman Soils: Distribution, Importance, Variability and Management. Texas Agricultural Experiment Station Bulletin B-1372. Texas A&M University, pp. 23. Van Genuchten, M.Th., 1980. A closed-form equation for predicting the hydraulic conductivity of unsaturated soils. Soil Sci. Soc. Am. J. 44, 892–898.

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